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. )