I agree with all your points, except the bolded.
If McDaniel gets canned, he will, IMO prove to be a horrendous OC on his own. He basically brought Shanny's system with him and added speed. Once teams figured out how to limit the offense, he has had no idea how to evolve.
He frequently gets locked into one mindset when calling plays and often abandons the run, despite his most dangerous offensive weapon being his RB. Not ideal when our best offensive player this year is our RB, and we tend to play more than a few games in bad weather every year.
Add to that the Shannahan system he would likey run isn't really what Josh excels at. It's more designed for mediocre QBs with weaker yet accurate passing styles. Tua, Purdy, Mac Jones. Guy that get the ball out and don't go off script. That's not Allen and I frankly don't think McDaniel has the ability to tailor an offense around him.