The NFL season isn't over. That tells me, or at least my gut, that Harbaugh isn't going to accept Michigan's offer until he fully vets all NFL offers. Word is that he's not satisfied by the presumed list of NFL suitors, but there's always the possibility of a surprise firing after Sunday's games, and only after those games are some of the NFL teams' offers to the 49ers likely to be finalized.
I'm willing to bet one or more NFL teams will beat Michigan's offer. However, they might not offer enough in compensation to the 49ers to satisfy them. And if they don't, then SF might not let Harbaugh go to any other NFL team, thus sending him into the Skunkweasels' open arms.
If Harbaugh already had made a final decision to go to Michigan, there'd be little reason for him to wait until after Sunday's games, since the 49ers already are eliminated. Their game vs. Arizona does have playoff seeding implications for the Cardinals, but I doubt that alone would keep Harbaugh mum. And if that's all he's waiting for, then official word on his departure for Michigan shouldn't come any later than Sunday night or Monday morning. The longer after that it takes for an official announcement, then the better the chances that Harbaugh stays in the NFL, in my opinion.