If there's one thing I took away from being a juror on a murder trial for 10+ weeks last summer, it's that the defense doesn't have to prove shit. It's 100% up to the prosecution to prove, beyond a reasonable doubt, that she did not deliberately disable the gun, and I don't see how that would be possible.
The problem is, the Defense has to make the Jury aware that the gun was disabled. The Jury doesn't have to buy that just because the Prosecution has to prove their case. I wouldn't believe that and I don't think most of a St. Louis jury will believe it either.
Depending on the Judge in a bench trial, the judge might get hostile with them if they are making that claim.
The Prosecution is not going to bring up "but the gun was accidentally disabled", the Defense is going to make the claim "she deliberately disabled the gun".
I wouldn't believe that, and I would look down on them for making the claim. Then I would want to convict them.
So yes, the Defense would have to prove to the Jury that she deliberately disabled the gun. Judges are not going to order a Jury to accept a Defense statement of intent as fact. No.
All a Prosecutor would have to do is ask her what I've already said:
"Are you making the claim that you deliberately disabled a firearm to confront a crowd that you claimed multiple times was making violent threats to you, and which you are claiming the [castle doctrine] defense for?"
Then I would bring up the two points I made above in the closing arguments, which is where they belong. This has the effect of prejudicing the jury just before they make their decision, and human beings remember the most recent points last.
Nope, I wouldn't even make that claim, it would backfire depending on the Jury. I guess if the Defense is completely confident they have a sympathetic jury... but the Prosecution gets 6 picks in a felony 12 person jury in my State, and in Missouri too I bet. That's 6 people who now want to convict the scummy lawyers because they tried to lie to them.
Juries punish for cockamamey lies. Nobody likes being lied to, it angers and offends them.