Love the "settled law" bullshit - no law is settled. See Dobbs and Bruen. Hint the Sullivan laws that Bruen strikes down were enacted in 1911.
She did not enter a government vehicle illegally. The government trespassed on her land illegally and created an attractive nuisance issue for her, which she resolved by securing the firearm in question. Had she knowingly left the firearm accessible on her land and it had been used to injure someone she would very likely been held accountable, especially if she was a permit holder which would lead on to believe she understood the danger of an unsecured and unattended firearm.
Law is based on a reasonable person's viewpoint. A reasonable jury of her peers (Texans) would likely find that she acted appropriately in the circumstance.
All of this assumes that the story is true that the guardsmen left the firearm unattended - the trespass need not be true to allow her to correct the attractive nuisance issue created by the guard but it does add to her case if they were trespassing.
The company commander involved needs to be visiting her, thanking her profusely, and writing an affidavit that exonerates her. She's the reason he's not in trouble: it's his name on the property book, and his ass if the weapon goes missing. She safeguarded it for him, he owes her.
What his first sergeant does to the Joe that left the rifle unattended is another matter, lol.