Try this:
http://jsc.defense.gov/Portals/99/Documents/MCM2016.pdf?ver=2016-12-08-181411-957
The page you're looking for is IV-26:
"(a) Mutiny by creating violence or disturbance. Mutiny by creating violence or disturbance may be committed by one person acting alone or by more than one acting together."
The Manual for Courts-Martial is the guidance utilised to interpret the UCMJ. Quite clearly, if a single person uses violence in the course of intending to overthrow or usurp the lawful authority of a superior officer, he or she can be charged with mutiny. Now, granted, off-times what happens is that the prosecution piles up charge after charge after charge and tries to get the defendant to plead to a lesser charge in return for getting rid of the big one (no kidding; that's how it was explained to me when I was a juror in a courts-martial; there were many charges but, realistically, only a handful the prosecution expected to stick). I'm not sure how many single acts of mutiny have been successfully prosecuted but it is, theoretically, possible. Once more, though, that is US law. We're not sure what Starfleet's code is, though it appears as if it takes its inspiration from US and/or UK common law (though in the UK, you
do need more than one person to commit a mutiny).
There were a host of things they could have nailed her to the wall on but it appears as if they went right for the jugular. To me, that implies they really, truly thought that what she did was so heinous, so reprehensible, that only a firm, irrevocable sentence would restore the deterrent value of obeying the chain of command and following lawful orders.
But, having said that, we're really left in the dark as to how they presented the case and adjudicated it. There were certainly mitigating circumstances, but...we don't know what was said. I blame it on the writers. They wanted drama. They got it.