If you'll permit a restatement... it sounds like you surveyed the verbal output of the big names in the transhumanist/singularity space and classified them in terms of seeming basically "correct" or "mistaken".

This is certainly not correct. It was more like "providing evidence-based justifications for their beliefs, or hand-waving arguments." I'm not commenting on the truth of their claims, just the reported evidence supporting them.

(2) relatedness to the LW/MIRI cultural subspace.

I don't association with LW/MIRI cultural subspace is a bad thing. I'm a cryonicist and that is definitely also very much in line with LW/MIRI cultural norms. There are just particular positions espoused by MIRI and commonly held in this community which I believe to be both incorrect and harmful.

My first question: Am I summarizing accurately?

Other than the above, yes, I believe you have summarized correctly.

Can you spell out the mechanisms by which you think mistake-causing reliance on thought experiment is promoted and/or transmitted? Is it an explicit doctrine? Is it via social copying of examples? Is it something else?

It isn't LW-specific. The problem lies actually with non-analytic, or at the very least casual philosophy, which unfortunately has become the norm for debate here, thought it is not at all confined to LW. Reasoning by means of loose analogies and thought experiments is dangerous, in no small part because it is designed to trigger heuristics of proof-by-comparison which is in fact an insufficient condition to change one's mind. Finding a thought experiment or analogous situation can give credibility to a theory. In the best case it takes a theory from possible to plausible. However there is a gulf from plausible to probable and/or correct which we must be careful not to cross without proper evidence.

The solution to this, in the sciences, is rigorous peer review. I am honestly not sure how that transfers to the forum / communal blog format.