What is the law or precedent that requires a judge to accept the prosecution's proposed jury instructions before the trial even begins or a jury is even seated?
There is none, so there is no reason for Cannon to be over-ruled, and her removal is a pipe dream.
Jury instructions will be a key part of this case, because the facts are not in dispute, as far as I know. The facts as known by the participants in the trial, anyway. Many posters on here are very confused as to exactly who did what, believing the TDS media's claims. But Trump had copies of documents with classified markings at his residence long after leaving the White House (as did many other former presidents, and people who were never president).
The question will be whether it was legal for him to have them. He had a security clearance until shortly after he was indicted, he had legit reasons to keep copies of the documents. He was president when he moved the documents to Mar-a-Lago. The jury will have to decide how these facts fit in with the laws.
That is what the Democrats fear: An informed jury deciding for themselves, rather than twelve partisan hacks who already "know" that Trump is guilty regardless of what the law says.
