I also do not remember if you have to defeat the second Demon Wall before you use the switch, but I don't think you do. The switch should be behind the spot where you first encounter the Demon Wall.
I dunno, once I had defeated the first Demon Wall, the initial step I took was to notice if any changes had happened in that area. No activity though.
Then I discovered the switch and pressed it but only a message popped up and nothing was unlocked (forgot what was stated). I think it was only after I'd wasted the 2nd Demon Wall in the next room did the taht particular switch actually functioned. Hmmm...

But no matter, if u can take care of the first Demon Wall, the next one would be nothing anyway.
Also, if you move 2 zones away, the enemies will reset, but you are required to travel 3 zones away to respawn the chests.
Ah, my bad. Guess that's why restarting to end up at the save crystal is all the more favourable than having to travel 3 zones away and 3 zones back between each spawned chest.