I do, however, strongly suspect they're expressing an opinion on something they don't actually understand - and I don't think that's an unfair assumption, given most people don't - which would imply they were only doing so because "believe in MWI" is a local trope.

What is "actually understand" here and why does it sound like a dichotomy? Are you arguing that one cannot have any opinion about MWI based on any amount of understanding derived from popularizations (Eliezer-written or otherwise) which falls short of one being able to solve technical problems you list?

Surely you don't believe that one is not allowed to hold any opinion or confidence levels without becoming a full-fledged domain expert, but that does sound like what your argument is.