@ianS
Given the relatively tenuous nature of the similarity between the "prophecies" and certain of the allegedly prophesied events reported in the Gospels, it seems more plausible in these cases that the evangelists and Paul searched the OT for passages that might explain some of the more intractable material they encountered in their sources, than that they looked at these sources and concocted events to match them.
In some cases, the birth story for example, the dependence on prophecy is indeed clear, because mistakes in the reading of the prophecy turn up as "facts" in Jesus' biography. But in other cases that phenomenon is not the most likely explanation.
When you say "material they encountered in their sources" (meaning sources other than the OT), which other sources are they? Where do you say they got their messiah beliefs except from the OT and from their religious beliefs in general (e.g. Paul believed that in addition to using the OT as his source, he had also seen visions of Jesus and had communications from Jesus and God as his sources) ... which other sources are you thinking of?
If you want to say that their sources were that they must have been told about Jesus from earlier Christians who knew Jesus and told them first hand evidence of Jesus, then that is a complete non-starter, because the whole point throughout all these threads and throughout all the sceptic books and articles ever written is that in fact there is zero evidence that anyone who ever met Jesus ever told Paul or any anonymous gospel copyist anything of the sort.
What it actually says in Paul’s letters, and iirc in g-Mark and g-Mathew too, is that they were definitely taking their messiah beliefs from what "is written", "according to scripture", and because "God was pleased to reveal his son in me" etc. And indeed, as I just pointed out (and as Helms shows in complete book of the stuff), you most definitely can find those passages in the OT ... that's not merely a coincidence is it!