As to your first point, the bust rate for 1st-round draftees is typically around 50%. Yeah, we're taking a risk on his talent, but it's the same risk that every other team takes with every draft pick they make. So it's not an incremental risk.
Your second point makes a lot of sense, but I think that's the risk that's already factored into his draft stock. Obviously, if everyone knew he was going to come back at 100%, there's no way he'd have lasted until the mid-2nd.
A bigger concern I have is actually one that you didn't mention - that even if he does come back at 100%, you're not getting a 1st-rounder at a 2nd-round price. You're getting three years of a 1st-rounder instead of five - one lost to injury in whole or part, and one lost to the lack of a fifth-year option on 2nd-rounders. Given what good coverage CBs command in the FA market these days, that's a significant delta that we'd need to eat at exactly the time we'll probably be trying to shoehorn Wentz's new $25M a year salary under the cap.