That is so sad
That does look rather serious. Perhaps some sort of sepsis?
Without a vet, though, I'm not sure what you can do. I understand there is no vet available, though.
Can you get some of the basic medicines that people use on reptiles ? I would normally never advise "going alone", but what choice do you have ?
Can you get Baytril antibiotcs? Dewormers like panacur? Something along the lines of Flagyl for possible protozoan infection ? Many of these meds are also used for other pets, would a vet be able to give you some of that?
Without knowing the cause, this would be a shot in the dark, for sure. At this point I would try anything.
Other then that all you CAN do is keep your husbandry as spot on as you can. With only the light as a heat source you might have low humidity, see if you can bump that up some.
Hope your snake gets better.