In terms of how Teresa's DNA got on the bullet.
It's been shown that those calibre bullets stand very little chance of doing a through and through on a human skull (with brain matter to go through). If it did there would be many traces of calcium and phosphate, it didn't. It was shown even after bullets were washed that the bone fragments remained.
Says who? Zellner or forensic science? What Zellner says should be taken with a pinch of salt, because nothing she's doing is being independently verified, or checked for accuracy - we have to take her word for everything - unlike court testimony, where everything is examined and cross examined.
I'm no forensic scientist, but according to some of the experimentation I've read - it's actually quite rare to find any biological material on a bullet at all, let alone DNA, however I'd certainly give way to the opinion of an actual forensic scientist who told me I was incorrect there.
In terms of how the DNA got on it if not through the act of murder... Who knows, it's hard to explain without going down the route of evidence planting/tampering.
My thoughts? I don't think she was shot and the bullet has been made to fit a narrative.
This is the problem that we can both agree on, any alternative theory that explains how the DNA got onto the bullet - other than it passing through Halbach's body - requires a gigantic leap into conspiracy theory land.
If you don't think she was shot - how do you explain the evidence presented in court, by the forensic anthropologist showing the skull fragments which had damage consistent with that of a gunshot to the head?
The bullet was one of the most damning pieces of evidence which the prosecution had. As you've said, they confirmed that the bullet they're saying killed Teresa was fired from Stevens gun and has Teresa's DNA on.
The questions Zellner poses however are;
1. Why is there no sign of any bone fragments on the bullet?
2. How has the bullet managed to pass through two sections of bone, and the brain itself, when it's only a small calibre round?
3. Why was the bullet covered in a waxy like substances?
The theory Zellner has come up with is that the bullet was literally picked up from the garage floor, Teresa's chapstick was used on it which accounts for the waxy substances and her DNA being on the bullet. It also explains why there's no evidence of bone being on the bullet, given that it never was fired at Teresa Halbach.
Who has independently verified, tested and agreed that Halbach's chapstick has been used on the bullet? Because until it is verified it can't be considered as evidence - I could run a TV show and make anything I like up to make a name for myself, by using cargo-cult pseudoscience, phoney brain scans and shooting guns at animal bones.
Apply Occam's razor to the two competing theories;
Steven Avery shot Halbach in the head with his rifle (as per Brendan's confession) also backed up by the forensic evidence showing a gunshot wound to Halbach's skull, along with her DNA on the bullet, they found in the garage at the alleged murder site, they then moved her body to the burn pit and disposed of it.
OR
An unknown third party (almost certainly colluding with the police) shot Teresa in the head (that's undisputed because of forensic evidence) then they either found a bullet fired from Avery's gun, or used his gun to fire one, planted her DNA on it via some means (her chapstick if you like) this unknown third party then burns the body in Avery's burn pit, - without him knowing, then proceeds to plant all the other evidence, including Avery's blood, the Rav 4 and all the other things (because simply putting Halbach's DNA on the bullet isn't enough, if you're going to frame one piece of evidence - it logically follows that all the evidence was framed) All of this, without any actual evidence that anything was planted.
Which is the most likely and reasonable explanation - if you were a Jury - which one would you pick?
