I've read all 17 pages of this thread, to date, but I am still stuck, at the final door. I've seen discussion of temple keys but they haven't helped. It would be a shame after 7 hours of this awesome mod to just quit at the final door. :*(
EDIT: I'm an idiot. Edited spoiler to clarify for others.
Situation is this:
I have two Temple Keys:
1 from level 9 (was in an alcove on the left just before a blue haze. I remember finding it clearly); and,
1 from behind the Unique door.
Discussion in this thread talks about a third temple key when you read the sign "you need another temple key". The problem is, you DON'T get a temple key here. You use a destiny key to get to that message, and when you use a temple key as the message states (my 2nd of 2 keys), behind it is JUST a destiny key. The obvious answer is to skip this door... except you can't. The game forces you to go through this original destiny key door followed by Temple key door, otherwise a door behind another destiny key door will not open which leads to the final fight.
I've done this temple/destiny/prison key area on level 4 many times, checking all possible permutations. I've never gotten a third Temple Key and that's what I need. I've checked all walls (I believe) numerous times, both for switches and walking into them. I've even loaded an earlier save before I ever got to level 4, went there afresh where it says next to the blue crystal that you need a Temple Key, and I don't get one at that message either. It always happens like this. I can spend 50 hours looking for something and not find it, and all I have to do is ask for help (publicize my inability) and boom, I find it within 30 minutes thereafter.
I even found the Master Key door for The End by accident. lolz Excellent mod. 10/10. would play again.
Oh, and that 45 seconds to find the right way... only after I determined it wasn't possible, did I notice the solution. That was plain mean! lolz