Another narrative:
BL and GP arrive at the Spread Creek camp site. She works on her project and he hikes off on his own, taking the debit card with her permission to use if he needed anything, a ride back, food, supplies, etc. Since she is camping near the van, she has everything she needs, supplies, food, her phone, etc.
BL hitchhikes back to GP, getting two rides.
BL finds GP dead and is afraid he'll be charged so he takes the van and goes home to get help from a lawyer.
Does it really matter what the people thought of him when they gave him rides? They can say, "he didn't smell as if he'd been hiking for two days," but how else did he get up to Colter Bay? Probably hiked. So, maybe he's not a smelly guy, or he cleaned up.
Oh, he only had a tarp in a backpack. Weird, but people can be weird. They guy hikes around barefoot apparently. I think that's weird too. So what?
Yes, he used the bank card. Probably for food and gas to get home.
Yes, it wasn't his account. Lots of people in relationships let their SO use their debit card. It sounds as if they were sharing the one bank account. People can do that. My understanding is that it's not a crime to use someone's bank card unless they report it as fraud. If he says knew she was dead, but he didn't kill her, and he drove home using the "shared" bank account, he may still be convicted of wrongly using it, but using the bank card doesn't prove that he killed her.
I hope they have some rock solid evidence, but the remains were out there for 3-4 weeks, and that, I believe, is an estimate. The defense may be able to argue that she could have been out there for 4-5 weeks and BL was home by then.
The defense may want to do another autopsy but they are unable to because the remains were cremated.
I'm sure there are lots of ways to pick apart a circumstantial case.