I think these are sufficient evidence that this is the real Dumbledore, not the mirror showing Quirrell what he wants.
The sense of doom. I thought the magic-can't-interact was mostly just the strongest edge of that--e.g. (maybe "i.e." too) their magic could interact but it would hurt them enough that they don't try.
That could just be a feature of the True Patronus, which is pretty anti-death and especially anti-indifference-to-other-people's-lives.
Your point 2 is another thing I'm getting pretty suspicious of. Quirrell has set up a very long plan, and could easily have faked this effect with wandless magic, or an enchantment he could later dispel, all along.
Certainly, and in the actual situation, I would have done worse than he actually did. But, this kind of armchair analysis is extremely enjoyable, and a good way to improve your in situ skills.
Harry made some serious mistakes in chapter 105.
First, the parseltongue honesty-binding could just be Quirrell's (selective!) wandless magic--I mean, he just forged a note "from yourself" (and why do you even MAKE a self-recognition ("I am a potato") policy if you just forget all about it once you're in a life-stakes intrigue) so you need a lot of extra suspicions going forward. But assuming it's real... there are crucial questions Harry can now profitably ask, with his help conditional on getting immediate Parseltongue answers, along the lines of:
"Why did you set up this elaborate ruse instead of just asking me? Most of what you're saying right now sounds like something I would've probably agreed to if you were open about it, but no, you had to pretend you were dying and kill my friend, so it sure seems like you're planning nefarious things I'd rather not aid even at the cost of my life and the hostages' lives... does my CURRENT utility function actually prefer your planned results to the death of me and the hostages?"
(This isn't the perfect phrasing; for one thing Quirrell doesn't necessarily know Harry's utility function to high accuracy, for another Harry might have disagreed to the "open" proposal at weaker dispreference than "this is worse than my death". But something similar...)
Iff Quirrell is at all "innocent" at this point, he'd want to answer these, and never mind the "my policy is never to reveal that much or people will know I'm guilty later when I actually need to keep mum" stuff; these stakes seem high enough to outweigh any future similar dealings. If he's guilty, then just die like you'd apparently prefer.
[the only edits I made here after getting responses were to correct my spelling of "Quirrell", and this note]
This is similar to choosing strict determinism over compatibilism. Which players are the "best" depends on each of those players' individual efforts during the game. You could extend the idea to the executives too, anyway--which groups of executives acquire better players is largely a function of which have the best executives.
Efforts are only one variable here, and the quote did say "largely a function of". Those being said, look at how often teams replay each other during a season with a different winner.
Mentioning a similarity to past successful decisions seems like it qualifies as "constructing a more contextually specific argument than 'you'll understand when you're older'".
While this is on My Side, I still have to protest trying to sneak any side (or particular (group of) utility function(s)) into the idea of "rationality".
But the map is the map...