Spending a bit of time to catch up on the backlog of replies I need to make.
Here's a suggestion: find a platformer where you think the gravity is perfect and where the player avatar is about the same size as Pouchie. Then aim to replicate THAT. For example, maybe a Castlevania game?
I 100% understand holding off while waiting on better graphics. That's fine in my book. :)
I think the reason the unkillable enemy gets me is because it's at the exact same time that you introduce the hammer, and it feels more appropriate that you'd need it for the shell creature than for the mushroom.
Regarding music, I guess it depends on the aim of the game. If you're going for a 1920s cartoon feel, I'd say go digging around for royalty free stuff.