That is an excellent plot twist.

But also, that means that if they hide one of the doors, then Team Evil will fight through all of them and will not find the Gate. Now, how to do that? Painting crosses doesn't work anymore. Illusions to make the door invisible? Come on, that's a quinton we have here. It has True Seeing at will. But there is one weakness in Team Evil's method: they skip the doors they already fought through. Do they remember these doors? Absolutely not, they just go in, look to see if there are monsters left, and go back outside.

That leaves a hole: if the OotS goes through, what, the 13th door from the end and fights through the first room (they can definitely do that. Even if they're not as strong as Xykon+Redcloak+Oona, fighting half a dungeon when Team Evil was doing 5 per day is quite reasonable), then when Team Evil goes there, they skip it. No illusion here, and Pass Without Trace may be doable if they increase Belkar's Wisdom (alternatively, hope they don't ask Oona to scout in every dungeon).

Then, they do all the doors, and notice that the Gate wasn't there. Of course, Redcloak gets angry, and understands that it was all a shell game, eventually finds the backstage, and goes there with Team Evil, just in time for an actual ambush and the final battle.