You're saying things like "stash" for a year and "hold a roster spot", as if this is a Jake Ballard situation, which it's not. His contract is voidable after this season. Unless you honestly believe the Giants plan to pay him that salary and carry his enormous cap hit, then the most logical thought would be that they plan on using him for THIS season to see what he's got left, and then if he does have something, they can renegotiate a new contract.
If he doesn't show anything this season he won't get another shot. He passed his physical, that means he healthy enough to play at least. He was a starter for the Panthers at the start of this season before getting benched for skill, not injury reasons. It's not like he's on the shelf, unable to play or contribute.
In their weak division, an 0-4 start might not be a death sentence like it would be for most teams. Dallas plays the Broncos this week, and should logically lose, so they'd be 2-3, and if the Giants win against the Eagles, they're 1-4, only 1½ games out of first place. So this trade on the surface, as I stated already, has not much to do with the future or next year, it's all about fortifying an area of need to try to make a run in a division that is still wide open even for an 0-4 team.