Replying to my own thread after a round of second tests
It seems if I move sims in unrelated households, not the ones that were randomly split apart/moved, it does not cause the game load error afterwards so it does seem to be related specifically to that bug/issue.
secondly, the sims that were moved out have no option to be asked to join household or ask to join household from their original household. I tested this with mortimer and cassandra and neither could "ask to join household" with the other sim. the option simply didn't exist even with high/max friendship. not sure if this is because cassandra is a teen ? (never tried asking to join household with a teen or child sim before) or if it is part of the bug.
thirdly, the split from household and move menu seems to be so broken, both in terms of almost all households being missing from the list to join and the menu itself freezing after trying to move (tested this with cassandra, unclear if this is an issue for All sims or just the ones impacted by this bug)
