His decision to land at MIA cannot be questioned as he had the authority. All that has happened is that he has lost credibility and trust with his employer. This is the true issue at hand.
This is a non sequitur. UA most certainly can question the Captain's decision to land at MIA (they obviously have) despite his authority to do so.
Clearly any employee who loses the confidence of their employer has a major career problem (true in any industry).
We don't know that he has lost UA's confidence though. In fact we don't much at all about the detail of this case and as some posters have been saying, we may be flogging a dead horse here.
Never has so much been made by so many out of so little detail.