The thing you found in the water was poo. It is not uncommon for MHDs to soak and then poo in the water. Make sure you change the water and clean the bowl anytime you find it soiled as you MHD needs clean water for drinking and can get mouth infection from drinking the water it has used for a toilet. This is actually a good sign since it would indicate that you MHD isn’t impacted and is passing food.
Two crickets a week isn’t enough for an MHD under normal conditions but if you are treating for parasites it may not have a normal appetite. Have you tried nightcrawlers yet? They seem to be a favorite of most WC MHDs. It is important to get her eating well so that she can stay healthy.
The black on her mouth could be a number of different things. It is possible that it was old shed. If she was dehydrated when/before you got her this would be very likely. It is also possible that she has been rubbing her nose against the glass or sides of her enclosure. This could have happened before you got her. It is also possible that she has an infection. If it is old shed you can prevent future problems by keeping the humidity at 70-80%. If it is from nose rubbing you can put paper around the tank where she may be trying to get through the glass. If there is a visible barrier she won’t try to get through. If it is an infection she would need to be evaluated by a vet and probably treated with antibiotics. The scales probably won’t grow back but the skin should heal if she receives proper care.
As for the weight, I couldn’t tell you if that is skinny or not without seeing her and/or knowing how long she is so I could compare her to my girls. Let me know how long from nose to vent and I will check that.
Hope this helps and I too hope she lives. If she doesn’t take to the nightcrawlers let me know and I will see what we can suggest to help her.
Marcia