Well...either they didn't ask him because they could not in their wildest dream think that someone would come in as a manager of this incredibly well run club and put everything into turmoil (which would have been very irresponsible), or they did ask him and there was a misunderstanding. I am not saying Moyes lied as in
SAF: Will you make any changes?
Moyes: No,no,no,no
but there could have been a miscommunication. Maybe SAF said something like "Everything is there for you. You just need to carry on" and Moyes said "Fine". This is ofc quite simplified here for the sake of the argument
Another way things happened could have been that Moyes initially didn't think he would need to make changes (when he signed) and honestly agreed to a smooth transition but when he started working he thought that he had to do them for various reasons. Considering the coach-replacements for instance, I would think he realized he was put in a dilemma: Either he would have had coaches that were better at his work than him himself, and whom the players would look up to more than him, or he got rid of them and hire people who he knew he could control and keep below himself in the hierarchy.
Your quality suggestion "willingness to take advice" was a really good suggestion.
I agree that the board should be critizised, and they will, I think. Since United is a public company they will be responsible for the bad results financially. Maybe SAF as well, but it's impossible to know how much input he really had, even though I agree that from all one can read, it seems he had a big influence. But nevertheless, the board are the ones formally responsible. This is also why it was never going to be ok to underperform for several years because "you are rebuilding things and making things the way you want them to be". You can't do that just because you have a stubborn and unadaptive manager. Not if you have a public company. You can make investments yes, but they need to be truly motivated. Financial results down due to big player investments --> ok. Financial results down due to an unnecessary change of essential staff and training routines --> na-ah.
Given that, Moyes really blew it IMO.