Jameis Winston’s contract becomes a $107m deal on March 16th trigger date

They actually can. They could either work out a new deal before that deadline or sign him again after he becomes a free agent. I don't think it's likely after the end of the Atlanta game and I think Winston wants to have a shot to start which DA isn't going to give him. But, if he can't find that, he could be signed to backup again here.
Oh, sweet lord above. How high does the writing on the wall have to be? 10 feet? 20?

They drafted a guy last year to groom as the next backup. They have been busy getting out from under Jameis' original 'starter QB' contract for over a year, ready to let him go. No amount of over-analysing an entirely overblown final play of the season changes any of that.

He gone. The plan has been that he be gone since at least the point they selected Haener. If not before.