While I do believe Alexander VI was the one with the actual strategic vision, I wonder if Cesare made the choice to back della Rovere under the influence of his disease/poisoning and perhaps his stage of syphills as well. I mean, why on earth was he naive enough to believe in della Rovere?
Which leads me to think, was there any chance for Cesare to consolidate a lasting Duchy of Romagna had D'Amboise won the past election or had Piccolomini lived a few years instead of mere weeks?
I've never really thought of this, but his deteriorating health could be a factor in making such a poor decision.
One can wonder whether Cesare's career would be better off had he remained a Cardinal. After all, despite all dangers of the Roman politics, cardinalate was much safer and more stable position than Condotierro-Prince (for example, our friend della Rovere did not lose his position within the College thorough the entire Borgia's rule). Cesare might not have become the third Borgia Pope, as his father intended, but he'd remain a powerful figure.
You've just remind me of very colorful description of the 1316 Conclave in Maurice Druon's novels: Cardinals Napoleon Orsini and Collona Bros putting curses at each other during the proceedings