Great Old One
So, I'm glad, because I find part of the objection to this to be... torturous rules-mongering, no matter what Jeremy Crawford thinks.
I think they tried to keep things simple, but like stealth (to which it's partially linked), it's an area with so many edge cases that it probably has to be specific rulings in many cases.
In the rules Ready appears in the "actions in combat" section. However, Cast Spells, Help, Hide, Search, and Use Objects are all things characters can clearly do outside of combat, so merely appearing on the Actions in Combat list doesn't mean you can't do it other times! While Crawford has his take on it, the rules a written don't explicitly prohibit it.
I use the same reasoning here.
The stronger argument, though, is that, "I ready to pull the lever as soon as the Evil Duke enters the room," should be a perfectly cromulent declaration for your character's action. Any rules interpretation that does not allow for such needs to go into the ashcan.
If that's not enough, we can note that in the case of ambush, the Readied action won't happen outside of combat. Bob says that he intends to start combat. Jill readies to go after Bob. John readies to go after Jill. Sarah readies to go after John. None of the readied actions will happen before Bob starts the fight! It's fine!
This is exactly the way I reason as well, it's the right way to synchronise, and for me the cherry on the cake is that it rewards players who actually plan for their ambush in a coordinated manner, while at the same time allowing it for their adversaries if the PCs are not careful.
For those who are supremely picky, we can note that this can all backfire spectacularly if Bob doesn't get surprise.
Yes, I think that's one of the beauties of the readied actions if they are used properly, they need to be specific, and because they need to be based on something perceivable by the character, it avoids a lot of shenanigans and metagaming about triggering them on technical game elements like "when it's his turn" or "when initiative is rolled".