You're saying that behaving as though an observer exists means it exists?
To answer that phrasing, taken as it is, not necessarily, much as the very limited scenario that would be required for it to not be the case is a bit eyeroll-worthy to bring up. To answer a question that you might have been actually asking if that question was asked honestly, if something behaves as though it actually is an observer, that certainly counts as evidence that it meets the criteria required to accurately call it an observer. Frankly, it's quite unreasonable to claim that we should just ignore such evidence, as you seem to think that we should when you tell us, apparently without any real basis, that observers are merely unexamined assumptions, unsupported by evidence. Naturally, valid evidence might maybe be able to be presented to the contrary, in any particular instance, but if you think about it even a little carefully, you'll find that disproving all observers to another would be an inherently self-defeating venture, because for another to be convinced by whatever argument, they would have to be an observer for the argument to mean anything.
There is no difference between the sense of there being an observer and being an observer.
At last check, there certainly can be. The former could count as a potential additional property of the latter or a false notion of some theoretical being that was not an observer. In practical usage, there's not much, though.
Unfortunately, for that to work, you are going to need a God! It doesn't matter how complex the brain is, it can't revoke monism.
Again, why are you invoking monism like that? All it's really sounding like is that you're invoking something that you simply don't understand.
Natural selection cannot create an actual observer. It's a physical impossibility unless you want to go back to dualism.
And this just goes back to the ever present need for you to... define "observer." Otherwise, the only valid way that this can be taken is that, despite your protests to the contrary, you seem to think that an "observer" requires some non-physical element, which would be known as a "soul" by more than a few others. Either way, under a materialist's definition, observers certainly can exist under materialism and trying to demand something not material to be the case for it to be an observer is inherently nonsensical. As it should be, frankly, given the nature of the word "observer."
One brain is aware of the illusion. The other isn't. The first has more options because it isn't just locked into objective awareness, reinforcing itself through constant referral to thought. It can utilize objective awareness but it's not dependent on it.
In today's increasingly stress-orientated world, this probably creates a survival advantage.
The second brain will likely react to the notion that there is "something it doesn't have" because it is so locked in its programming loop. It can't step outside objective reality. All it can do is tell itself stories about how objective reality is all there is, and can you prove this or that to me, trying to prove to itself that this illusory and artificial mode of interpreting perception is the only way there is!
I don't feel like dissecting this right now, honestly. Not least because it sounds like pointing out things that should probably be considered relevant would cause you to retreat further back into religion/blind faith mode. So with that, I'm done for now.