I think your achievement system is okay, got about half of them.
The community buttons work on my side, but as it should be obvious Steam has nothing to display yet as the game is unreleased (this is Valve's job to set these up not Nifflas').
So far I haven't noticed any screen change delays, so it might be a computer thing.
I was playing around with the overlay and noticed that every time I close it, the cursor assumes it's position before opening the overlay.
I've attached an image of Knytt Underground on Steam's library with achievements and screenshots.
EDIT: I somehow messed up the screenshot, I've uploaded a better one.