Yeah, I found my way, but it was not about keys, I just missed a few rooms in level 3, so I went back and found the Letter.
More glitches (v1.6) :
Did anyone mention the glitchy fairies in dungeons? It doesn't crash the game, and it refills your health fine. The hearts surrounding Link are more numerous, it makes the game lag a bit, and after the hearts disappear, two extra fairy sprites appear randomly.
In dungeon 3, a hint about burning a tree above the armos, has glitched tiles on the 3rd line. First line is "BURN THE TREE AT". The entire text could be ywanked back a bit and have first line say "BURN THE TREE AT THE", and it will free up space for the two next lines.
The Ghost that says the hint "ABOVE THE NE. RIVER" also has glitched tiles. You could avoid that by using three lines of text instead of two, and it would let you say "NORTHEAST" instead of "NE".
One thing I bumped into that may confuse people. Because you can have Potions without having the Letter first, it's very likely that you have a Potion (Red or Blue) in your inventory before finding the guy who says nothing and needs the Letter. If it's the case, the guy doesn't react. You have to actually drink the Blue Potion to have the item slot turn back to Letter, and then the guy will reveal the shop.
There is a room in Dungeon 9 (First Quest), first column (from left), 4th row (from top), that has 4 Traps in it. I don't think there's any possible way to push the block and open the south door without being hit at least once. Are you comfortable with that?