Thanks for your reply. "Typical bugs in RPG Maker games" has become such a dominant thought pattern in my mind that I might not be able anymore to recognize all Mystic Quest Easter Eggs. ;)
I really like the intricate dungeon design, and it's great to see how some areas are surprisingly linked to each other. Since I could only see the red treasure chest in Ice Pyramid 3F and not the teleportation panel leading to it, I thought it was inaccessible, but I'll keep the possibility of teleportation panels in mind from now on in case I'll stumble upon another seemingly inaccessible treasure chest.
Is it safe to continue my playthrough or should I wait for the next update? I can't figure out exactly how the trophies work (how the requirements are set up, how and how often the game checks for the completion of requirements), so I don't know if the trophies can "fix themselves" later. Although I even conserved a save file before opening the last treasure chest in Hellmouth Hill, that might not help later with regards to the related trophy, dependent on how it works.
Soon, I'll need to make a tough call and decide whether I should spend all my money on stat boosters right now. :D