For the animated movie (and possibly the comic book, as I haven't seen it). Caiera tries to rescue a little girl from the spike attacks, but the girl ends up disintegrating in her hands, presumably from the anti-Spike weapon. Caiera was holding the girl in her arms the whole time. How did the girl get hit but Caiera not?
Caiera was protected by the power of the oldstrong, which is the same power that allowed her to fight the spikes without being contaminated by them. Basically, she was immune to fire, but the child was not.
Why would the Illuminati build a self-destruct sequence that could destroy an entire planet if they were sending Hulk to a planet?
They didn't. It turned out that the explosion was a bomb planted by Red King loyalists in order to kill the Hulk.