I think it's optimistic to think you could get a 1st. Probably also that we could use them to trade up. And even if we did both, it's pretty optimistic to think that whomever we then drafted would turn out any better than either of those guys.
I wouldn't be opposed to trading Nyquist or Tatar (or even both depending on who we draft, and where our other prospects are at) for defensive help, maybe even for a pick if someone was dangling a decent 1st-rounder and there was a kid I liked still available. Dekeyser I wouldn't move for anything I think we could actually get (unless we were able to flip Nyq/Tatar for some major help on D). I haven't been too impressed with any of our kids this year.
And what would we do with that cap space? Clear a bunch of cap then throw a fortune at Shattenkirk, only to immediately start bitching that we need to dump him so we can go after the next guy? We're not going to turn the team around with UFAs. Not likely to turn the team around overnight either. We don't need to desperately dump cap, especially not cap that will be gone within three years anyway.
While that's true that we couldn't just force him to IR, he does have chronic knee issues. I'd think any doctor would say playing further poses a significant risk of further damage. I think if he were decide he didn't want to play anymore, we'd have no problem getting LTIR for him.