Ok. Let's take one reaction preset at a time. Attack of Opportunity is the primary reaction everyone has.
AOO presets - All Enemies (Default); Selected Target(s); Off
Basically, players will standardly want to have AOO trigger every time any enemy leaves melee without disengage. However, there may be times when they don't want to waste it on certain enemies. Example: Don't want it to trigger on goblin. Want to save it for the bugbear who might deal more damage to my mage if he gets past me. So, Selected Target(s) would allow the player to, during their turn, set AOO and then pick only those targets they want it to work on.
1) To me it sounds horrible and tedious. What if I preset AOO to hit targets A and B, but then something happend during the combat (before my reaction) and now I wouldn't want to aoo target A, but instead would aoo target C if thet come out of my range. I PAUSE THE GAME AND CHANGE THE PRESETS. Which defeats the whole purpose of having pausless "flow". Then the situation develops and I do it again. For each character. For each possible reaction. For each remaining enemy. And if I don't do it, I feel like I don't utilize the full potential of the character. No, thanks. I'd rather react to situation each time.
2) If you can only change presets of a character during this character's turn, it defeats the whole purpose of reactions. Reactions to what enemy does before it's your turn again.
No. You don't pause the game to change presets. You change them on your characters's turn. It's not flawless, but it would cut out the need for A LOT of pauses and prompts.