I don't think that any of this is likely to happen but to me what you are talking about is similar to either baseball's minor league or to the loan system in European soccer leagues. In the end the team doesn't get any sort of compensation for injured players, that's just a risk you take. The idea behind them is generally to get younger players with potential some playing time to develop them. But if we are theoretically sending Chad Kelly to the XFL to get some playing time, we are most likely not planning on him being a major starter that year.
But quite frankly I just don't think it will happen.
I don't think so. Kelly goes and plays in the XFL for a couple years he comes back he's still Colt's property. (That's even presuming his contract allows him to go play for the other league, I'm guessing it doesn't.) Even a wildly successful XFL player likely isn't going to improve his stock enough to make him attractive trade bait for the Colts.
He's better off seeing where the NFL thing takes him. The Colts kept him on the roster for some reason, perhaps he's in line to be the #2 guy in the future. In that case he's simply a play away from NFL playing time.
If not he can just stick around in the league for a while and push his past behavior further behind him and eventually when the Colts either let him go or his contract runs out he might get a call for a backup job somewhere else.
A lot of the value of him sticking with the Colts is just in proving to NFL teams that he's grown up. After that he can let his college resume and pre-season play try and get his foot in the door someplace else.