Well first, I think there's an obvious reason that Ryno was passed on and they let Quade take the manager's role. No one in their right mind thought this team would sniff .500, nonetheless be in contention. Too many aging, lack-luster vets. Too many questions with the kids. Too many high-dollar, minimal production salaries to do anything with. When Henry gave a press conference regarding hiring Trammel as the bench coach for Lou, he went on for a good 5 minutes about how he thought the Tigers set him up to fail and that it was a dastardly thing to do. Had Ryno been hired to manage this team this year... it would have been almost the exact same situation Detroit put Trammel in. So, instead of putting in the vaulted hero to the masses in charge right away... you put in the oaf that has paid his dues in the minors as a manager. In the end Ryno goes and gets some experience with a different philosophy (expanding his horizons and knowledge), and probably improves his ability to manage. Quade gets credit for managing beyond the "interim" bit. This means he'll probably easily land a coaching job in the majors once he's let go.
So you cover your fall guy, while setting up the "hero" to be a more capable manager... with a potentially better roster to do it with. If you also bring him in with the notion of "we're going to rebuild starting with our farm system" the observant fans, and the media, will give Ryno a pass that they would not if he failed to provide a winning team under the guise of "this team had the talent and pieces to compete in a weak division if everything goes right and the manager keeps them from falling apart".
As for Hendry, you don't get to rebuild 3 times without winning a championship and/or string together several playoff appearances. Rebuilding 3 times in 8 years is quite ridiculous... especially for a team that should be one of the elite teams in the league if Free Agency acquisitions were better handled, obsession with certain prospects kept certain beneficial trades from happening, and bad decisions not been made on hiring managers who couldn't hack the Chicago media spotlight... or break away from failing, ancient strategic philosophies.
Hendry would still be GM if he hadn't signed Soriano, Bradley, or Fukudome out of desperation... hired Joe Girardi over Piniella... hired Terry Francona over Dusty Baker... and had a backup plan that made any kind of at any time during the Wood/Prior injury plagues. I'm not going into the nuances of trades or FA signings that didn't pan out, but looked like good deals without the hindsight. These are moves that were bloody obvious at the time.
As for the timing. You keep him long enough to get the draftees signed and do the head work for potential waver-wire deals. You get him out long enough for others GMs that may be interested in the job (current or former) to begin doing what they may need to do to become eligible for it. I'd think the goal would be to begin interviewing in late September, and during the playoffs... while hoping to have someone in place by the end of December. However, I wouldn't expect a current GM (or assistant GM) to leave his position until after his current team's off season acquisitions are pretty much done.