Why is there a button for Attack of opportunity?
Because you can turn it off and also to make sure that your character prioritizes the correct "auto-behaviour" if there are multiple options. Pretty sure you need a bonus action or something similar for AoO to trigger (can't swear on it).
I do agree with you that parts of the UI should be overhauled, and also the parts about the spells/sneak attack and buttons for party sneak toggle.
No.
AOO is one of reaction, separate from bonus action.
But like bonus action, you can use only 1 reaction in a turn (or in between your turns)
For example, you might want save the reaction by not doing AOO and use it for a shield spell on yourself.
