1. Only A3's own Gold members can obtain a seat assignment in advance of check-in - this applies for both Y & C. However, if you check-in 48 hours before you are likely to find very good availability anyway.
2. A3's layout is substantially the same as most other airline's A320 aircraft. You could possibly secure an emergency exit if you're quick to check-in, otherwise you'll find pitch and width fairly standard for Europe. There are no preferential seats for *G.
3. You should put your wife's UA number in the booking - no chance of her getting correspondence if the booking is attached to your e-mail.