As far as I see it we have three possible CB's who might step in if Coleman stayed at FS - Eric Kelly, Faggins, and Wright (if healthy). I have mixed feelings about leaving Coleman at FS in that situation when he has a potential playmaker backing him up in McCree, and he himself had 7 int's as a corner last season. Depends on how many reps Coleman gets as a CB in practice I suppose.
By the way, I second your opinions that I would hate to see this circumstance come up, but at the same time I unfortunately think that we'll have atleast one injury in the secondary. Hopefully whatever plan the coaches have in place is a successful one. We do have plenty of depth at the corner position now, IMO.