Regarding one, two and three: shouldn't we, in any case, be able to make an educated guess? Am I wrong in assuming that based on our current scientific knowledge, it is more likely true than not? (My current feeling is that based on my own understanding, this is what I should believe, but that the idea is so outrageous that there ought to be a flaw somewhere.)

Two is an interesting point, though; I find it a bit baffling that there seems to be no consensus about how infinities actually work in the context of multiverses ("infinite", "practically infinite" and "very big" are routinely used interchangeably, at least in text that is not rigorously scientific).

Regarding four, I'm not so sure. Take cryonics for example. I suppose it does either increase or decrease the likelihood that a person ends up in an uncomfortable world. Which way is it, and how big is the effect? Of course, it's possible that in the really long run (say, trillions of times the lifespan of the universe) it doesn't matter.

Regarding five, I guess so. Then again, one might argue that big world immortality would itself be a 'very bad thing'.