PocketFrog is 100% source & 100% free. GapiDraw uses a dll and will cost if you are a serious developer otherwise free. Both have forums here. See www.pocketfrog.com and www.gapidraw.com
Screenshot from PocketFrog? here's a shot from my soon to be finished game: .

Difference between WinCE & PocketPC etc. WindowsCE is the underlying operating system & PocketPC is a presentation layer onto that.
Why assembler, cause you can get better performance over compiled C
Get evc++ installed & then try PF or GD samples