The demo extra can be found in the central part of town. A gate that is closed with a guard should be open once all world fragments and tf forms are collected.
You're not stuck i would say, and there is only on switch here. On the three stone plates above you just need to move down under the stone bridge thing.
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!
