That 2nd pass just finished (I let it complete, this time). And indeed, this one works. (I think .mov headers are always added when the file is done only). So, yes, it may be a crash-recovery support file. It had an extension of .prob3 (problem?), so it may indeed be used as a second pass, to potentially pick up where VEAI left off on an earlier crash.
Tl;dr: matter solved. It was human error (namely mine). Thanks for shedding light on this 2nd pass recovery pass.
EDIT: The pauses haven’t stopped, but, fortunately, don’t make part of the output file. (Could actually be the pauses are possible flush-moments, to forcibly commit data to disk, intermittently; and in case of ProRes, those be HUGE chunks). It’s all coming together now.