Yes, the game autosaves, although you will start all the way back at your house.
And yeah, I nearly gave up when fighting the last guy in the colosseum too.
The strike can be gotten separately. Got the strike on Tampa, Florida (twice!) but so far, the closest I've gotten to completing the game is being about 50 points short of reaching level 10...
I wouldn't give up the uncontrollable cannon at all – it forces you to slow down, and makes it the relaxing (and yet infuriating) paradox of a game that it is.