I don't think Jerry will hire a young coach or someone who has not either performed well as a coordinator or performed in a HC capacity. Jerry has ot hired a coach like that since the Dave Campo days, and I honestly don't think he'll start now. Besides, I agree Garrett has playcalled himself out of contention. Garrett is part of the current culture, and making him HC will only keep the current culture.
The other part of the problem is Holmgren, Shanahan, and Gruden are West Coast Offense coaches. It takes a couple of seasons for an offense to really grasp teh WCO, and this team is built to win now. Jerry won't want to wait for the offense to pick up the nuances of the West Coast Offense with 2 or 3 mediocre seasons.
Harbaugh makes sense as does Frazier and Rivera. And you can bet Jerry will do his due diligence and give Cowher, maybe even Dungy a call.
If the worse happens and Garrett does become HC, then I hope Jerry gets a good OC and DC, because let's face it Garrett will become a puppet and let Jerry make his hires. Maybe bring Charlie Weiss in here for OC. Dom Capers got promoted to DC in Green Bay and Mike Trgovac is no longer DC. So Jerry will have to find a solid DC who knows the 3-4 defense.