I had to do this with my vine snake who was in really bad shape when I got him. I just worked with the swallowing motions the snake made and rubbed gently down if that makes sense.
As for euthanasia thats for each person to decide as long as you feel there is a chance than its your choice to keep trying. I have brought animals back from near death while others I knew just didn't want to fight anymore and had them put to sleep. Personally I would keep trying as long as I felt the animal had a will to live but that's just me.