Not sure if I ever played that lot. I know I never tried to build apartments there though.
1) Removing the wall is the only way I know to get the stairs to work. I was looking for the mod you mentioned, but can't find anything anywhere. That stinks I would have liked that myself. :p Anyway, the only stair mod I found made by Lord Darcy was this one.... http://www.modthesims.info/download.php?t=303905 not what you were looking for I imagine.
2) I always fill the open portion as much as I can, then delete one row of tile. That always works well for me anyway.
3) It is either your buyable mailbox, all research I have done on this says that most of them were not converted for apartment life, which messes with the mailbox. I found this, don't know if it is the one you were using or not but they all seem to have the same issue.. https://www.gamefaqs.com/boards/946800-the-sims-2-apartment-life/45259317
OR you might have more then one copy in your downloads folder (they are sometimes included with lots), that will mess with it even if the version you are using is up to date, Or if you moved the mailbox while building, it will not convert.
I read about other people having modular stairs mess up the apartment transition. Especially if you were using staging or something to make a L shape. People say this is not allowed, I haven't tried to make apartments in ages, so I can only tell you what I found.
