I’m sure you’ll agree that’s a tougher question, Bricker, although it is a fair one. I honestly don’t know that a Dept. of Justice can just stop defending an extant case that’s being adjudicated, can they?
But let’s say under Romney the law has been ruled in error some way by a lesser court and then Pres. Dean is sworn in and instructs the AG (either his newly confirmed nominee or the existing Romney era guy) not to appeal that decision. I think this is probably not improper. Same answer if PPACA had not already been upheld in SCOTUS and a 2017 administration decided not to defend it. Then I think it bounces back to Congress to fix or scrap the law per the specific judgement.
Same with selective prosecution, I think. I recall that the Bush administration had definite political priorities in the selection of cases to prosecute. I recall many of their choices infuriating those of my political stripe, but I don’t believe that was ever seriously considered unconstitutional discretion.