personally I would amend son return now and paper file parent now with claiming the son. I have done that in the past with no problem. Amended returns turns take so much time to process. I would not file without son then amend with son. I won't file a return I know is wrong.
go to schedule D and their is tab for sale of residence. If no 1099-S issued and under the reporting amount you can skip it BUT I always report it to be on the safe side. Clients don't always bring in the 1099-s
a duplex has a joint wall on the same tax lot. In my area we can't split for a sale. Maybe it depends on counties and states. Yes it is two separate home but still joined.
I would separate it into 2 rentals, not four because you can't sell just one half of a duplex. If you are living in one part of the duplex then I would separate that one out. But if all four are rented do just 2. also allocate land and buildings.
Gift of over 15,000. gift tax return needs to be filed. Her basis is fathers basis plus any gift tax paid.
Now is the time to hunt down the fathers basis.