I do not think the Pats have any shot at getting Tua. He will be long gone before the Pats pick and I doubt they have enough to move up where he will likely go. If he is available for Pats in round 1, it likely means teams are completely scared off about his hip and in that case picking him in round one would be a huge risk.
Assuming Brady is gone, I think Stidham will get a chance to be the starter and they will either bring in a veteran as backup/tutor or possibly even let Kessler have the back up role.
The finances are such that next year Pats can not afford a starting veteran QB from outside the organization.
If Brady leaves, next year will be a re-tooling year. Expectations will be appropriately low and the pats may finally get :
1) high draft picks in 2021
2) higher priority in waiver wire
3) more cap space
With smart management it could be 2-3 years before they have the talent to make another run.