It's also unobvious which mushrooms can be jumped on. By too linear I meant left-right-left-right pattern of getting powerups.
As for codes, I found the last set through cheating - that is, the one hidden near the water.
Something about how you changed the bird's sprite on the cutscene to the 'romantic' ending had me laughing out loud.
Ha, didn't notice that! A Sonic bird :)