Every point you made is debatable.
Taysom is a dynamic athlete but QB's don't have to be dynamic athletes. Taysom can run like the devil but he has flaws in his throw game. Were the flaws worked out? Some seem to think they were based on one half of a game vs Nebraska.
The answer to why BYU re-recruited Taysom is simple. Why not have him back? If he can start great but if he can't maybe we have a serviceable backup. Your deep thinking into the issue really wasn't what went down. At no time was Taysom told he had the job.
I'm not arguing that one should start over the other but discounting Tanner's ability in the throwgame is stupid. In that sense he has more upside than Taysom and is a better fit for what Detmer wants to do.
May the best guy win.