I think the withdrawal of the cash is two things, and this is all my speculation here:
I bet he paid for the gas and everything else via cash so it couldn't be traced. I believe that they were still trying to figure out when and where he bought the gas?
I also believe that JP was at the point of believing that the police/media were harassing him - I think he actually convinced himself, and obviously family members, of this so he took the cash out on that reason.
I don't doubt that he died in that fire because one, where would he go? 7k won't get you far, maybe to Mexico (Canada is out of the question because how high profile this is). He won't board a plane, but you know maybe he can and he had gotten a fake ID/passport? I dunno, nothing makes sense and I could be totally incorrect. He won't be able to go very many places with this story reaching around the globe like it has. 7k really isn't a lot of money to reinvent your life, and under the conditions he would want things? I really do think he died in that house that day. Again, I might be totally wrong here.
I do believe (*speculating still*) that his sister received the money and for several reasons. The 9-11 call of her.. well, I don't feel like she was worried. I feel that it seemed forced, staged even, right down to the "He has never hurt me, he has been abused in the media" bit. I am trying very hard to be compassionate so please don't take this as a bash Mods.
I think that the warrant for AP's phone speaks volumes. That the warrant also had "any text message, voice mails or deleted messages" was a flag. She was also named a suspect in that same warrant.
Again, speculating on my part. I should probably stop thinking about this so much.