A - his beliefs on MWI have no bearing on his relative importance wrt the future of the world.

B - when you say "defensible", you mean "accepted by the clear majority of scientists working in the field".

Karma scores in this thread suggest it falls in reference class of "arguing against groupthink", which ironically increases estimates of Eliezer being a crackpot, and lesswrong turning into a cult, possibly via evaporative cooling.

No, that's really not borne out by the evidence. Multifolaterose's posts have been strongly upvoted, it seems to me, by a significant group of readers who see themselves as defenders against groupthink. It's just that you have been voted down for refusing to see a distinction that's clearly there, between "here is a complicated thing which nevertheless must be reducible to simpler things, which is what we're in the process of rigorously doing" and "here is a magical thing which we won't ever have a mathematical understanding of, but it will work if we play by the right rules".

"Karma" is the only definitionally supernatural item on that list - it is defined to be not reducible to nonmental mechanism. The others are merely elements of belief systems which contain elements that are supernatural (e.g. God).

Yes, the concept of "karma" can be reduced to naturalistic roots if you accept metaphysical naturalism, but the actual thing cannot be. It's the quotation which you can reduce, not the referent.