Who else should we blame for Manchester United problems because it seems like one problem is connected to another. Whether it's internal or external problems, this club has changed completely in the last few seasons. As you said, total change must start from the top, namely the Manchester United board itself, there is no common vision in rebuilding the club.
Maybe starting with the club directors would also be a good decision because they are the ones who make decisions about who to appoint as head coach. The problem is that they take too long to replace managers and give too many chances, first to Solskjaer, then to Ten Hag, even though it was obvious they needed to be replaced. Now the same thing is happening with Amorim. He is managing the club very poorly and Manchester United's performance last season was the worst in their history. But they are keeping him as head coach, and I think this is a big mistake.