It would probably need at least five or six in the loco pool, as they would need the same modifications as the Chiltern ones to work with the DVTs, plus some extra mods (interlock? Not sure what it is but the dedicated 67s have acquired an additional socket). DRS only have 10 of them for their own use specifically, plus whatever Chiltern and TPE aren't using, so they may well not have enough to spare. They will also probably be more expensive to lease as DRS are paying lease costs on them and will want to make a profit on top. The 67s are owned outright and fully paid-for, so are costing DB nothing in hire charges.
I would expect the Chiltern ones to go in the next franchise and be replaced by DMUs of some sort, although that might be delayed a bit now. WOuld be surprised if they last more than anther five years or so though.
TPE may well be stuck with the sets until whenever the lease ends, but I wouldn't bet on them lasting beyond that. An 88 would be of no use unless the whole of the route was wired (the diesel engine would not be powerful enough for York-Scarborough, and I would doubt if it can provide ETS on diesel in any case).
68s and 88s are quite different, so any 'conversion' of locos would be major work, if even possible. Yes, they look superficially the same as they have the same cabs and bogies, but the bodyshells are different (the 88s are longer), and no doubt the load-bearing structure will be completely different.