How to get 6 ending?
So from what I can tell: (And obvious Spoilers for key location etc.)
1 Don't move.
2 Reach Trowzer without the key after the time limit.
3 Reach Trowzer with the key, but after the time limit (you can grab the key later. It's Upwards, first left, past the minefield, then up until you are in a room with four exits, all but the bottom one surrounded by 2-wide lines of mines. The key is in one of the three adjacant rooms, but it's rng which one)
4 Get the key and reach Trowzer in time
5: Collect all keys (including the one you have to collect as the first-it's in one of the rng rooms, apparantly)
6 (Edit: Thanks to Garry_T_Fox) Collect the dead-looking key in one of the rng rooms. You CANNOT collect any other key before it. Return to Trowzer's room and go right, past Shovel Knight until you find a big key at the end. Bring it to Trowzer.
I know how to get ending six (Even more spoilers)
What you have to do is get the same key that you have to collect first, then you have to get back to Trowzer, go right (where Shovel Knight is,) and ;eep going right until you hit what is normally a dead end, but now holds the fabled "right key." After a cutscene plays, just return to Trowzer. (Which is made much easier with the power of the Right Key. )