I liked to think I knew the basic framework of the game perfectly, but last night it turned out that I didn't know where you have action windows when attacking an enemy, probably because it never came up in any relevant situation (or the answer was obvious all the other times that it did and I didn't remember them).

So I didn't know if this plan would work (it does!):

Play Knight of Minas Tirith

Activate its response, engaging an enemy and declaring an attack (step 1 of the attacking framework)

Action window opens!

Play Quick Strike, targeting the enemy with a character armed with Rivendell Blade (step 1 of the attacking framework)

Resolve this attack, including reducing the enemy's defence (steps 2 and 3 of the attacking framework)

Resolve the Knight's attack, taking advantage of the reduced defence. (steps 2 and 3 of the attacking framework)

Know your action windows!

That would be neat but the Knight's attack isn't part of the framework, it's a card ability that has to be resolved in total in one step - there are no action windows inside of that resolution.

No, NathanH has got it fine. The card allows you to "declare it as an attacker," which sets up the usual series of attack resolution steps, including action windows. Same goes for Quick Strike. This is just like when an immediate attack happens from an enemy revealed during the staging step of the quest phase. Even though you're in the staging step, where there is usually no action window, this sets up a regular defense and you can take actions at the usual places during attack resolution.

Yes but Knight's text says "and resolve it's attack" as part of the ability. Other effects will say "makes an attack" or "declare as an attacker" without stipulating that the attack has to be resolved in that card effect (edit: Quick Strike does say to resolve the attack).

This section from the FAQ looks relevant and looks like it takes the action windows out for Quick Strike as well:

Q: Can characters with the ranged keyword participate in an attack declared through the card Quick Strike (CORE 35)?

A: No. There is no opportunity for other characters to join a Quick Strike attack. One character is exhausted to pay for the cost of Quick Strike, and the effect is that the exhausted character is immediately declared as an attacker against the target enemy. The card’s resolution does not allow for a standard declaration step in which other characters can declare​

Yes but Knight's text says "and resolve it's attack" as part of the ability. Other effects will say "makes an attack" or "declare as an attacker" without stipulating that the attack has to be resolved in that card effect.

While I kind of see your point, I'm not sure why resolving the attack would preclude the normal action windows opening. Resolving the attack presumably means, go through all the steps of a normal attack, which include an action window.

(By the way, Quick Strike has similar wording.)

Hmm I see what you mean. I guess it's unclear then if the windows are there, or it's clear to everyone else but me that they are there. If so, that allows for some fun shenanigans with Quick Strike as mentioned by OP.

The limitation on Ranged joining Quick Strike doesn't have anything to do with the action windows, it's because the ability specifically allows a single attacker. It's not "declare an attack", it's "pick a character and declare them as an attacker."

We know the attack process is largely intact, because you go through the standard process. I don't see any reason to think that the action windows somehow vanish. And even if there's a question, precedent has generally pushed towards action windows existing even when they're not explicitly stated (such as with Gollum's riddles).

I think the OP's trick is fine (if a bit combotastic )

I carry these with me where ever I go to play LotR:

http://www.orderofgamers.com/downloads/LordoftheRingsLCG_v2.pdf

(see pg. 5)

I know it doesn't exactly help with your scenario since it is a nested function of the player card, but, it does detail action windows very clearly when you can do them!

An attack ALWAYS has the same action windows, no matter when or why it occurs. So many aspects of the game would fundamentally break if there were not action windows at the appropriate places during attacks. The fact that a player card caused the attack is irrelevant, think of it as kicking off the framework "resolve a player attack against an enemy" steps. It just happens to be doing it during the planning phase (or whatever phase, in the case of Quick Strike). An attack is an attack, no matter when it happens. The same is true for enemy attacks, which is one of the few ways that you can initiate player actions during the staging step in the quest phase (when an encounter card causes an enemy to make an immediate attack).

If they decided to come out with revised rules for Attack and Defense that broke those events out of the action framework and had their own referential sequence, it wouldn't surprise me.