WHAT THEY SHOULD REALLY DO IS SAY THIS:1) All prepped actions go off just before their trigger (as traditional).
2) If the trigger was a spell being cast and it has a cast time of 1 action, then it still goes off, but AFTER the prepped action (assuming the spellcaster hasn't been dropped). If the casting time is longer than 1 action, it is interrupted and fails.
At which point you have to add even further exceptions for Attacks of Opportunity (which explicitly do happen before the triggering action), which is not good.
It may not make sense to you, but might be because you're over-focused on an edge case that (a) doesn't really come up in practice, and (b) is actually pretty easily handled by creative use of the surprise round rules.
Also, the door counts as a purely defensive action, so it gets to happen first. One way to think about it is like this: The rules allow for blocking, but not for you to land your punch before your opponent's punch as a reaction to it. Effectively, it's an assumption that a purely defensive action is just a tiny bit faster than an attack.
The problem here isn't the rules, so much as the way you're framing them for yourself.