Hey, I just started testing your mod out and wanted to mention that there is one more thing/known issue that might be worth adding in, so that people are aware in case they encounter something similar.
I couldn't add one specific sim as a founder (it worked for others).
The messages I got in the console:
Firstname is now the founder of family A!
adding to family...
making heir...
Firstname Lastname is not in, or included in, family A
you need to add either famX or incX before you can add heirX
Of course, adding him to famA as per the suggestion didn't help at all.
I figured out it was because his name was identical to a sim who passed away and got culled a few generations back and it seems the mod attempted to set the culled ghost as the founder instead of the alive sim.
I decided to run the command using sim's id, and learned that sims.get_sim_id_by_name (First Name) (Last Name) cheat also returns only the id of the culled ghost. I got the correct id from mccc logs and then it finally ran successfully :)
edit: On second thought, temporarily renaming the sim might have been much faster xD