The buttons you can use could easily be in the UI as shown in the Composer's screenshots. In other words, you get the "React" or "Skip", and in the UI below only the Reactions you can use would appear with flashing buttons so you know they are available.
You use Sneak Attack during your turn. Enemy triggers AOO. Only Melee Attack appears on the UI hotbar. Sneak Attack does not.
Next round. You don't use Sneak Attack. Enemy triggers AOO. Sneak attack appears with Melee button for reaction.
Enemy triggers AOO for Battlemaster who didn't use Maneuvers during her turn. React? All maneuvers appear on the hotbar along with standard Melee attack button. No messages needed.
As for Counterspell, enemy mage casts Lightning Bolt. React? Only Counterspell appears on the UI hotbar. You don't need to designate what level spell. Lightning Bolt is a level 3 spell. If you hit Counterspell button, boom. You cast it and stop the lightning bolt.
It could be just that simple if they use the prompt method.
