So about Negative Man's powers:

It seems like he's indestructible like David Dunn or Luke Cage, and has superhuman resistance to higher and extreme temperatures. Electricity also makes him suffer, but not die like anyone else.
I think his "deformed" aspect is not burning-out, but the effect of the radioactivity that he absorbed.
That would explain how he survived the rocket crash and why you can see - in the origin flashback - the flames barely "scorching" him.