With regards to restarting challenge missions, here's a tip: Just get yourself killed if you fail. Get a Game Over and choose continue.
Beats canceling the mission and restarting it with more load times
Yeah, falling into the lava seems to be the easiest way to restart.
Also, I have no idea how to do this Health race. Of course all the trophies are set up so that you want to do them all. lol
Holy crap, and as I say that, I got the gold. Damn, that's a rough race. Tip: use the gravity kick to get to as many way points as possible, master the slide race at the end. In fact, if you know you are going to fail, you should just turn around and practice it a few times.
Also, 4 bucks is really pricey for the DLC which is maybe half an hour to an hour of content for most players. The costume would be nice if you could replay story missions or had some other incentive to go around the world.
And strangely enough, I still haven't levelled up my reputation to be able to buy more skills. I have no idea what they're saving it for.