From what I gather from BKs interviews, BVGs scheme, and other information posted here I think that our defense will be versatile depending on who we play. Against spread teams we are likely to see smaller faster players that can play man, defend in space and tackle (which I see more nickle base groups). Other teams like Stanford will require bulk in the front 7 so we could still see bulky 3 or 4-man fronts. Lots of possibilities but I could see him playing FS or CB in BvGs scheme. Maybe not 3 downs each game all season long, but he could play IMO.
As far as a slot, I like Crawford much better at slot than I did Mackenzie.
Regarding cross-training, has anyone actually trained to play both sides of the ball under BK at ND? Several have made position switches, but I cannot think of any that have played both sides during the same season.