When a HC offers a position coach a job on his staff, it should be pretty obvious to everyone in the room that the commitment is for at least one season. I do not think it needs to be said in late January that the expectation is the person accepting the position will remain in that position at least until the upcoming season has concluded. He was, after all, hired to coach and recruit for a football team, whose primary purpose is to play (and win) games.
I also sincerely doubt any position coach offered a coordinator position who would otherwise accept it would be dissuaded from doing so over the use of the words -- "first year commitment". A position coach who takes his commitments so seriously that he would not take an appealing job, but only when the HC expressly asks for a one-year commitment and he agrees (instead of just inferring it given the nature of the position offered)? That seems like a strange code to live by.
And isn't it up to TWill to be "clear enough about what he wants to do" during the interview process? Not that I think this one matters much either way, but it seemed like an odd side to place the responsibility to communicate in this instance... UM needed to be clearer with TWill about TWill's future coaching goals outside the program? I guess Manny could have conceivably asked TWill something along the lines of, 'If Gus gets hired as a HC this season and comes calling with a DC position, will you leave'? But (a) what assistant coach honestly says no to that question; and (b) what HC decides not to hire an assistant coach based on that answer?
There are plenty of areas/events/instances where Manny rightfully deserves blame for the failings of the program, but this seems like one of those times where a couple of opportunities (Gus's and TWill's) happened to align against the program. It's just the nature of the game.