Oh, please do. This is a fascinating subject and it's hard to reach any conclusion about it.
Well, my opinions on the topic have been said to death already. But as a summary
1. Paul. He could have been:
A) actually making stuff up, as you say. It's not like someone could check whether the vision actually existed or not, and he does claim point-blank that he performs miracles. Unless he's delusional, he's a con-man. There aren't any other two versions for someone to tell people, basically, "but you saw me do miracles." And the incentive certainly is there too.
B) he could be talking about something else than we think. A big problem with reading Paul is that most people seem unable to put away the things they know from other sources. E.g., they read "revelation" and think "ah-ha, his vision", because they've read Acts too. E.g., they read "apostle" and think "ah-ha, one of those 12 guys who hung out with Jesus for a year." E.g., they read "gospel" and think, "ah-ha, coherent narrative of Jesus's life and deeds." But Paul generally doesn't say any of that. It's people filling in those blanks with something that COULD be what Paul meant in the first place, but also COULD be something completely different. In fact, Paul could be spinning in his grave so hard over what the gospel writers did with his message, that if we could wrap him in copper wire and put a magnet on his tomb, we'd have free energy for ever
C) Paul could be simply paranoid schizophrenic. There's again the fact that he may actually think he is doing miracles. There is the fact that taken literally his epistles describe several schizophrenia delusions, including referential and textbook Cotard. There's the fact that he didn't have just one hallucination, but communicated with Jesus and got answers afterwards, and has another revelation later and acts on it. There's the fact that his pattern of writing a passionate 20 page letter about Jesus, yet at the same time showing no interest whatsoever in learning or sharing almost anything about Jesus, is the kind of thing that is occasionally seen in paranoid schizophrenics. Etc. So he could actually be a very honest guy, and honestly conveying what the voices in his head were telling him, but, you know, you'd have to be even crazier to take the hallucinations of a crazy guy as evidence anyway
2. Q. Ah, Q. Q is a proposed source for the stuff that is in both Matthew and Luke, but not in Mark. Both copied almost word for word from Mark, and Luke didn't even change the order much (Matthew did occasionally rearrange stuff), so when other stories appear almost word for word in both, well, you know, it's highly improbable that something like that would happen by sheer random chance. So some smart people came up with the idea that both used a source -- in German "Quelle" -- hence, named Q.
Nobody actually saw Q, though. (Dan Brown seemed to think the Vatican has it, but, heh, the Vatican would probably sell half their souls for something like that.)
For the same reason that Luke seems to just copy in order, we generally take the order in Luke as PROBABLY the order in whatever form Q had. So when you see people saying something about chapter and verse number from Q, it means that verse and chapter from Luke. It's really that equivalent. You know, in case you run into something like that.
Now before I get any further, you have to understand that we can't reconstruct the whole Q. Nobody saw it, nobody knows even how long it is, etc. ALL we can say is that a couple of stories from both Matthew and Luke are from that. We don't know what else was in there. We don't know if it was supposed to be a whole gospel as we understand that today (i.e., a narrative about Jesus's life), or used in parallel with a real gospel. Etc. So all I'm about to say from this point is actually only what can be said about the pieces we have, not about the whole Q. And that can make a whole lot of difference.
So for a start, it seems to be a collection of standalone sayings, sorta like the Gospel Of Thomas or on the other side of the globe, the Analects of Confucius. Jesus doesn't enact elaborate symbolic actions like in Mark for example, but just delivers some smart line and that's it. Actually unlike Confucius or Thomas, it is a bit different in that it doesn't just start with "Jesus said X" or "Peter asked Y, and Jesus answered Z". It can set the stage a bit for Black Hole Sue... err... Jesus to deliver his punchline. But at the end of the day, that was the whole payload, really.
Second, there seems to be no birth or resurrection narrative in Q. Or at least the pieces we have, well, they don't cover that. It could be a pretty big thing IF it was a full gospel, AND had no resurrection, well, that would be kinda significant. Unfortunately we can't really tell either of the two, and anyone who claims otherwise is pretty much just extrapolating way more than supportable.
Third, it seems to have a more Galilean flavour. The stage for those smart lines Jesus says is generally set in the more rural Galilee. Again, it's unclear if the whole Q is like that, in which case maybe it was collected in Galilee, or just Luke and Matthew simply chose a more Galilee-centric set to counter-balance the more Judah-centric stuff in Mark. If I wanted to have a messiah that represents ALL Jews, I'd want to balance it like that too. But ok, we don't know what's in the rest of it, if anything at all, so let's say it's got a more Galilean theme.
Fourth, it seems to be MUCH less about an imminent apocalypse. Now ASSUMING that it was used as an independent gospel, AND that indeed the rest of it wasn't apocalyptic either, then you can argue all sorts of nonsense about it. Like one such claim I've seen was that it must be earlier than Paul, because obviously the apocalyptic twist came later. Never mind that in stuff like the DSS we see an apocalyptic tradition long before Jesus was even born, or that all along what they expected the messiah to do was start the messianic age, i.e., the end times. Again, usually such guesses are really unsupportable, and again one can't really support either of those two initial assumptions either.
Fifth, we don't really know anything about what form it was in, who wrote it, or indeed if it even was in written form at all, when, etc. There is a lot of postulating, implicitly or explicitly, but as I was saying, usually on fundamentally unsound premises and lots of confabulation. That is, if it's even supported at all. A lot of people just start from stating what it was, when it was, and how authoritative it was, and skip that boring part of actually supporting any of that.
But anyway, in case you didn't know some of that, that's Q.
3. Mark. Well, Mark itself is a big question mark. Again, there is a lot of stuff taken for granted about it, but actually a lot less that can be actually supported.
For a start, the date. Traditionally it was pushed to be even before Paul, like the other gospels, but then someone noticed that it kinda talks about the destruction of the temple, so, you know, unless Mark was clairvoyant, it's unlikely it was actually written before 70 CE. (Almost all good prophecies are written after the fact

) So then basically it all was moved to, "ok, ok, then it was written in the 70's". But that's actually only one end of the interval, and it's kinda not good historiography to do that. Actually Mark could be anywhere between the 70's and the 130's, assuming that Marcion's gospel is a derivative of Mark. So, you know, the actual supportable dating is a much larger interval.
Mark was actually traditionally thought to be the last, or at least third gospel written, and be basically a summary of Matthew and Luke. Well, it's still impossible to fully disprove that. It could be that Luke actually copied and rearranged Matthew, and some word group concordances kinda weakly point that way, and then Mark just chucked away all the fluff that wasn't in both gospels. Well, it could be, but almost nobody takes it seriously any more. The far more plausible version is that Mark came first and Matthew and Luke copied from it.
Mark is also... weird. THE number one theme in it is reversal of expectations, i.e., literally irony, and a general theme of failure. This is also one reason to make it be right after the destruction of the temple, which Mark would take as THE apocalypse, but as I was saying, we don't really know that. I mean, 135 was just as big a catastrophe, for example. But a possibility that some people are starting to at least consider is that MAYBE Mark wasn't trying to write a gospel after all. Maybe Mark is writing a cryptic work of satire at the messianic expectations which had just failed. Sorta like how "The Prince" turns out to be satire, not actual advice to princes on how to be a ruthless power-hungry son-of-a-witch.
(And if that's the case, now that's probably another guy spinning in his grave

)
Mark is also writing something that doesn't look like a collection of independent stories about Jesus, but a highly coherent story, with a good structure and masterful use of foreshadow and layered metaphors. It also may have parallels to Homer. I.e., IMHO probably he wasn't asking any witnesses, but just writing the story from scratch.
The fact that he makes a complete hash of the geography, customs, religion, etc, and seems oblivious to language nuances (although some stuff does seem to come from a translation from Hebrew or Aramaic to Greek), also isn't exactly pointing at his getting the story from witnesses from Palestine. There's stuff that not many people from the area could be confused about, yet Mark makes a hash of it anyway. So, yeah, that stuff ain't coming from peasants and fishermen from Palestine, but from a highly skilled, highly educated Greek-speaking author.
Now why would a Christian just make up the story of Jesus instead of wanting to know what actually happened? Well, for a start, not all Christians are honest, and not all Christians are sane. But there is also the explanation that MAYBE he's actually writing symbolic stuff. In fact he even has Jesus say point blank that he speaks in parables so the uninitiated won't understand what it's about. So Mark may actually be writing about some stuff he believed, but is basically writing a symbolic version of it. Sorta like when Aesop tells you about a mouse talking to a lion, he's not exactly trying to tell you about a lion and a mouse, if you get my drift.
Strangely enough, Mark's Jesus is also a hush-hush secret ops Jesus. He does miracles all over the place, but tells people to not tell anyone about it, and they don't. He sends his apostles to do miracles all over the place, but tell people to not tell anyone about it, and they don't. And it culminates in his empty tomb and an angel telling the women to tell people about it, except they still don't, and the story ends there. In fact, you could make a Mark drinking game. You take a shot of tequila every time Jesus says to keep him a secret, and then go get your stomach pumped or you might die
So basically Mark is a literal conspiracy theory: he has thousands of uncoordinated people keeping a secret, even though it's against their own best interest to do so. You'd think if they know Jesus is real, and they know it, they'd at least want their friends, parents, children, brothers, etc, to profit from that knowledge and go to heaven. But no, TEH JOOS don't tell anyone about Jesus, because he told them not to. Go figure.
Why would Mark do that? Well, no idea. Maybe it's just reminding the initiates to not go blather about what it really means. Or maybe he knows he's making up industrial-grade BS, and is afraid people will ask how come nobody heard any of that. Well, he heads that off at the pass by saying up front why they never heard about any of that: the Jews are keeping the secret, that's why. Either case isn't entirely encouraging me to take Mark as actual history.
And so on, and so forth... I really doubt that I could do such a broad topic justice in one message, even if I stayed up the next week writing this.