Nobody actually believes the cops killed Theresa Halbach. Avery doesn't and his defense team didn't and I don't think you could reasonably conclude from all of the information out there. But that doesn't mean the cops didn't try to pin the murder on him.
I think the easiest explanation is that someone murdered Halbach near Avery's property shortly after she took pictures of the van and decided to hide her vehicle in the salvage yard. When her disappearance was reported and the cops knew that Avery was the last person to see her alive, they assumed that Avery did it and went about making sure he was convicted, regardless of whether or not he was actually guilty.
I believe the simplest explanation is generally going to be the right one in these cases.
You are suggesting that a third party and the police, acting independently of one another and at different points in the timeline, both conspired to frame Steven Avery, who was completely uninvolved in the murder of this woman even though he was the only one who we know interacted with her on the day in question, and the only person living anywhere near where her remains were found that actually had a personal connection to her.
That, to me, is a stretch. To say the least.
What's more likely?
A vast conspiracy involving multiple parties acting independently?
Or that this one guy, who has an extremely low IQ, has a history of odd, violent, deviant behavior -- which very likely got much worse in prison because that's how prison works -- murdered this woman and then did a terrible job of attempting to cover up the evidence?
This, again, is really frustrating because you're complicating something that is really simple.
If some third party killed Theresa Halbach and wanted to dump the car somewhere, the Avery's salvage yard would not just be a reasonable place to do so, it would be the sensible place to do so. It doesn't have to be someone "framing" Steven Avery or even knowing anything about him.
Same thing when we get to the police. The conspiracy doesn't have to be "vast" and it doesn't even really have to be a conspiracy. Like his lawyers said at trial, this could've been 2 cops, aided by a system that rewards convictions, instead of truth. And these were guys who already didn't like Avery, thought he might've been guilty of another crime (despite all the evidence he wasn't), and figured he was guilty of this one too and wanted to ensure he didn't get away.
Trying to pin the murder on Avery doesn't even require malice on their part. They might think he's guilty and think they're doing the right thing. But they're still wrong to have done it. But that explanation, to me, is a lot simpler than the one you'd need to come up with to explain away all of the discrepencies with the state's case.