1...ACE only affects the user and I have found it better to play with it turned off as you will be able to have confidence that what you call...sticks until you change it. The drawback in turning ACE off is not being able to set your tempo/rebound strategies at the beginning of the game and having to do it once the games starts.
2...I set all my strategies and call my own plays as I concur that the AI will make some baffling playcalls and w/o the "show user playcalls" and/or player communication turned on for both offense and defense, you will not know if any changes are made with ACE on and playcall set to "Auto".
3...If you do turn ACE off....be prepared to be fully immersed into the game...offensive strategy changes, playcalls, tempo, etc as well as defensive strategies, matchups, and etc. BE prepared to make these changes, manually, the moment you see it's not working as the AI will exploit mismatches and slow adapting users.
That's just my experience and advice with having ACE turned off. Hope this helps a bit.