It's on sale for $25 this week. Better do it asap if you want to trade.
You can't use coupons toward preorders except at time of purchase. Any idiot could have realized you could just pay for a $60 game with just $25 subscription if that were the case.
Trade your games in, receive the credit on a trade card.
Make your steam card purchase in a separate transaction.
I'm not actually aware of any restriction of having the trade and purchase on the same transaction. The restriction in place is any purchase of gift cards using Gamestop gift...