You are an evil, evil young man. Every time I play this levelpack it shows me just how deviously I must make my levels. I am going to need to contract in outside help for this.
Shelley Shack has a small bug, when Keen's on a pole a Shelley can push him along, even through walls and doors. Aside from this, it was flawless( Or at least I don't think I found any. I got to the secret level without knowing how for example.)
Playing this is like swimming through treacle; it's one long hard slog after another. The levels are absolutely *huge* and it looks like that's not changing anytime soon.
You need more shortcuts for easy difficulty (If it helps I'm pretty sure Keen 5 supports different starting points for Keen on different difficulty settings.)
So all in all, my goodness this is a boot to the head! Keep up the good work, it'll certainly be something to see when finished. (And no, I didn't find your sneaky secret passage, I hardly found half o them I don't think.)