The game's pretty good, but I definitely think adding the checkpoints was the wrong way to fix the game's difficulty, as it makes the mid-game sex scenes less meaningful, since the healing isn't so special anymore.
What you should've done to fix the difficulty is to reduce the hitbox to a small-ish square on the character, as it's awfully annoying to play a bullethell game where your hitbox is a tall rectangle, especially with all the horizontal jumping we have to do.
EDIT: I guess I've got a little suggestion, too: change it so the imps change color or expression or something after they bounce off the ground, it's annoying to have to gamble whether or not it's safe to stand next to the spot they're heading towards.