Yes fine gravel is bad for your axie, as they can eat it and get impaction from it. Please dont remove all gravel as it will contain a bactira which helps the water cycle and be otherwise heathly. I would surgest getting some pebbles (remove axie while doing this) and put them on top of the current lay of gravel you have. This will then keep the bactira and you shouldnt eat any gravel eating. The best size guide is something that wont fit in there mouth, say 1inch or more. But try and get it so that the smaller gravel is covered!
You might be best to leave her main tank as it is and put her in a bare bottom holding tank for a few days, then you will be able to see if she passes any gravel.
Best thing to do with feeding is to just keep offereing her, she will soon eat when she gets hungry, axies can go quie a length of time without feeding if they need to, or in the wild if food in minmal.
What temp do you have her at, at the moment. The hotter is it the more stressed she will be...
The only good reason to give her a salt bath is if she has any signs of fungus.
Anyway good luck and keep us updated.