Trying to Understand Many-Worlds Interpretation Better

[Angrboda]

I'm just a layperson with no real knowledge of physics, but here's my understanding of it.

If MWI is true, does this mean that anything that is possible has 100% happened somewhere in at least one of the infinite parallel universes? Is it really possible that there is a "me" out there somewhere that has done the most heinous things that one could ever imagine?

According to my understanding, no. All possible worlds evolve according to the same wave function equations, so only those things for which there is a quantum mechanically consistent evolution can occur. All possibilities occur as a result of the wave function collapsing in all possible ways that the wave function can collapse up to the Heisenberg cut.

Must quantum immortality logically follow from the truth of MWI? Are we really each an immortal character in our own universe?

I don't understand quantum immortality enough to say anything. I suppose if immortality is consistent with a wave function collapse evolution.

What bizarre things on the macro level have been witnessed in this particular universe that we share (if we even do share the exact same universe) that may hint at the truth of MWI?

I think this comes close to a misinterpretation. There may be no macro level events that suggest the existence of alternate evolutions of the wave function aside from the quantum level laws themselves, but I don't really know.

What does this tell us about the possibility of Christianity or other religions (or gods) being true? If they are impossible here, would they still be considered impossible in all the other universes no matter how many parallel universes there may be?

I don't think the existence of a god in the traditional sense is compatible with ordinary quantum physics, so no, gods would not be a feature unless they are already a potential feature of this world line.

What does this tell us about the ability to predict anything in the future given that MWI is true?

I'm not sure I understand the question. From what I understand, MWI in some sense implies the B theory of time, that all possible evolutions of the wave function have already occurred. The most numerous world lines will be those that are most probable according to the laws of quatum mechanics, but if the B theory of time holds, all the possible world lines exist simultaneously (if that concept even has any meaning).

Finally, how do you feel about MWI being possibly true?

I'm a fan of MWI myself. But I really know too little about the physics to really have an opinion.