Update, I had to remove all modular stairs and replace with the normal staircases in each apartment. What's up with that? It would not stop saying invalid apartment until I replaced all stairs with the Maxis premade ones. This did not clear up the mailbox problem, though. I moved a Sim in to test (after removing the buyable mailbox from downloads) and got boolprop interactions on mailbox in a different language, so I had to put that mailbox mesh back in game so I can read English commands if I use mailbox to cheat etc. I have no idea why the game won't convert the mailbox to an apartment one. Since removing any custom mailboxes didn't work but made things worse.
