All the jury has to be convinced of, is that Patrick Frazee murdered Kelsey.
That’s it.
Even if they don’t believe parts of KK’s account, there is plenty of other evidence that incriminates him.
Don’t believe he burned the body? Fine, it doesn’t change the rest of it.
It doesn’t take away the cell phone evidence, all his lies, the physical evidence that corroborates KK’s account (accelerant, burned plastic, blood splatter, etc), the footage of him returning to the house, and all the things that we don’t yet know.
A potential hole in a single piece of evidence (I don’t believe it exists in the first place), doesn’t blow a hole in the entire case.