Event is undefined, but it's not undefinable. We don't have to allow anarchy.
Truth, but we can't put the cart before the horse. Until "event" is defined, we can't allow enchantments to be revealed in response to them or it will be anarchy.
Not huge: there's precident from the movement steps with passage attacks, and there's a plan for normalizing those rules.
Exactly. Until "event" is given a definition, the rule should be that Enchantments can be revealed after a Phase, Step, or Activation. It is entirely possible for something to occur during a Phase that has Steps, in which case Enchantments can be revealed after any of those Steps even though it is during a Phase. It doesn't matter whether the Phase is Upkeep, Deployment, Quickcast, or an Action Phase. They're all the same for these purposes. If they contain steps, then you can reveal after any of those steps. If they don't contain steps (or an activation), then you can't reveal anything during the Phase.
I would certainly be willing to switch position once "event" had an acceptable definition, but until that time I firmly believe that any ruling relying on "events" would break the game.