As for feedback, the final place you need to go to for the harder-to-get of the two endings is probably the easiest location in the whole game to overlook. Maybe it would be nice to make it more clear that that's where the player needs to go in some way. Pointing them in the right direction at that final point in the game would definitely be nice.
By the way, after trading the three items away, you can still somehow give them to the slime dude and get his ending instead.
you mean, the dealer? Yeah, well at the beginning of the game I decided to draw an arrow sign saying which way to go at the first cross-path... you're supposed to notice the dealer in the little... room. Oh well, I guess not enough info.
As far as trading the pieces and still "dying", that's a glitch. I'll fix it in version 1.1...