I see it now. I think my reply to this will cover both anyway.
I think police have a pretty solid theory involving the FPs and they're trying to gather evidence around that, but it is difficult because of the amount of time that has passed and the fact that certain crucial things were not done in the first place. They are very limited in what they can do. I think that, as you say subsequently, they really want to solve this, so they'll throw the kitchen sink at it if they have to.
So I think they did it for a combination of three things: to see what other evidence might come out of it (so yes, a fishing expedition, but because of the particular difficulty of obtaining new evidence in this case). to apply pressure, and also because they are pissed off. If the FPs have lied about something crucial that has thrown this entire investigation out of whack, that's 7 years of work down the drain. I'd be pissed, too.
Oh 100% I think GJ wanted to solve this. I actually think it was the case that broke him, and that's why he ended up going way too far. But people have blind spots, and he had such extreme tunnel vision I can believe he missed things. If the FPs were involved, let's not forget, they fooled most of the country at one point. GJ is an unbelievable police officer, but he's still human.