You have some excellent points, but some things I think already have some answers:
A, B, & C) You're definitely right. This will be a new role and a lot of new responsibility that Gattis hasn't seen, yet. Of course most of the offensive staff have some influence on play-calling, play-development, and building the playbook overall, but exactly how integral Gattis was that at Bama is hard to say, and obviously isn't nearly at the same level he'll be responsible for at Michigan.
D) I would argue that Michigan's offense this year is as close to a "Bama-level" offense than in years past. Patterson is an established QB, with 2nd best QB rating in the B1G last year despite having a limited offense under Harbaugh / Hamilton. DPJ & Collins are good, established receivers, and Black comes back from injury and Bell will be a solid 2nd-year starter, as well. The OL may actually be the strongest unit on the team, with both guards and center almost certain early-round NFL picks in next years' draft. NO offense is what Bama has year in, year out. However, this is likely to be Michigan's best in terms of talent and experience in the past 5 years.
E) 3 most important positions for a Gattis-style offense: QB, OL, WR. Patterson is a clear dual-threat guy, so his skillset wasn't quite utilized last year, and this offense fits his attributes better. McCaffrey (back-up QB) is also a quick guy, so dual-threat fits him, as well. The OL had a ton of zone concepts last that align with the RPO mindset, so that tracks well. The biggest "missing piece" is the slot jitterbug type WRs. Michigan has about 3-4 true freshmen they'll look at to try to fill that role. Otherwise, I think between DPJ / Collins / Black, they are guys who fit well in just about any system.
F) I think it takes a LOT longer to fully install a new defensive system than it does an offensive system. Defense is almost always reactionary, so having to teach a new defensive approach for every single type of offense you face is extremely difficult. Teaching a new offense is most impactful on the QB, then the OL, since most of the other positions are scheme-agnostic, beyond some new techniques. I mentioned most of it in E, but I think a lot of the changes for the offense fit the OL and QB relatively well. Having a senior QB helps, too.
G) A fair point -- this article states that Harbaugh is taking the hands-off approach this year. I think it's fair to question that, especially in the scenario that you said that if things get shaky, how will Harbaugh react? However, if he is TRULY hands-off through spring / fall practice, I feel like it would have to be an insane level of incompetency from Gattis for Harbaugh to feel like he has to re-implement the offense. A thing in favor of Harbaugh actually being hands-off is the absence of the FB on the roster. They've eliminated that position and the main FB from last year (Ben Mason) has shifted to DT and added a bunch of weight during the offseason. So, if he's willing to give up his main FB to a primarily defensive role, I tend to believe he's letting Gattis run the show, though I am biased :)