I'm not quite clear on how it differs but it definitely don't work with the enter. Maybe you could try SDLK_RETURN || SDLK_KP_ENTER (the logical OR should work in an IF, otherwise make a seperate entry for a CASE)

I think the backspace is SDLK_BACKSPACE (or SDLK_DELETE)? I'll have to get back to you with definite answers.