I think JJ would hire Mike Holmgren. Holmgren/Favre was a successful combination. I can see Jerry thinking Holmgren/Romo would work as well. Probably not Gruden, because then if Gruden was successful it would look like Gruden and his staff moved from Tampa Bay and won because of that. The credit would then go to the coach instead of Jerry Jones, which was what really caused the riff between he and Jimmy.
I personally do not think that Jerry Jones would ever fire Garrett. He swallowed his pride when he hired Parcells, but I do not think he will ever do that again. He has a coach that he likes personally, and one that allows him (JJ) to be the face of the organization. The merchandise still sells well, his team is still the most valuable in the NFL, and he is still the GM. His toy may be broken, but it is still his shiny toy that he doesn't have to share.