Leaves breaking down!

That is not a nute toxicity of any kind... If you've got mushy leaves/stems, I think you've got a problem with root rot. Have you had problems with fungus gnats? They normally don't go after roots, the prefer to eat the decaying matter in the soil but when they run out of that, they'll start attacking the roots. Since it's an auto, I really DON'T want to suggest you unpot her to take a look at the roots (much too stressful!) - but she sounds like she's not salvagable anyway... I would get her out of the tent and take a look at the roots - there really isn't, as far as I know, any other reason this would be happening to your plant. Good luck...