Ah, I was there but then this ends the game, right?
Ah, not sure how missed it, I think I did try going there, but maybe I messed up. Still, the switch can't be triggered and I'm still blocked to access the extra chests.
Nope, it does play teh credits but you can leave and save afterwards so no worry about going there. There is some event in tehre after which xou can't save but that is optional and you also will be warned bevore.
The switch has nothing to do with the three extra chests. There are three hidden switches that open thoses gates to the treasure chests. The are very hidden. If you want the spoiler: They are hidden in the torches at the beginning of teh dungeonHope this helps!