Why worry whether there was or wasn't one of Sheila's fingerprints was on a shotgun? I would like to see someone pick up a shotgun and fire it with one finger.
For the sake of accuracy. So many things have been alleged as facts that turned out to be untrue. It is necessary to start from scratch nd determine what the facts are in order to then assess the case on the basis of such facts.
While it is true she can't have held the shotgun in one hand and rifle in the other Rambo style and that no shotgun wounds were present so the shotgun can't have been used by her or Jeremy in commission of the murders that doesn't mean there can't be some tangential value.
If her print wa son the gun it would have to be because at some point she moved the weapon or Jeremy planted her print on it but why would he do that? If she moved it that would suggest that at some point the gun had been in her way and needed to be moved which could be used to counter claism the gun was always stowed.