I do strongly agree with your last part on how perfect Trestman would be for him, however Bell was taken in the 2nd. Not 6th overall. And yes an elite back can make a difference. Up until they're injured, (and my guess is they're the most injured and shortest life position) or the game dictates differently, (still will have impact but minimal)
I'm aware it's a completely different scenerio but even if he lived up to AP's status, look at where thats gotten the Vikings...If we had a Cowboys oline I would understand it better. Just much rather take advantage at the rare spot were picking at and draft a game changing pass rusher, cover man or receiver.