add medium and large apartments to the apartment housing
Absolutely, why not ? Consider it "instanced housing"

Limit house ownership to 1 house per FC/account per Data Center (one account can own a house as an FC leader, and a house as personal)**
No.

add a small 2x2 inside gardening housing item. (if that can even be done)
Good idea tbh, lets people use gardening to make onions or whatever for selling.

add choices to the layout of the house, nothing crazy, just a "mirror" option (so small house with stairs on left side)
Big agree

Allow FCs to choose where to place the door for members rooms
Why not ? Sounds good

**Note on ownership changes:
When changes are made, send out emails to all effected accounts, that they must reply with what house they want to keep with location/server. All other houses will be demolished, released and owners will be refunded 80% of the value to the characters connected to them.
Certainly not. If you think they will do that, you're dreaming. And tbh, they shouldnt.