1) If you open your reservation on TP webpage you should be able to click on add. luggage (look for the suitcase icon). You can also add it during OLCI (at a slightly higher cost than if booking in advance).
2) idn
3) You should get 1 pcs as UA Gold. You will not get 1 pcs for all pax (unless the check-in staff is very generous; I wouldn't count on it).
4) No.
5) You need to check if an upgrade changes the checked luggage allowance. Often it doesn't. idn for TP upgrades.