I don't mean to rub anyone the wrong way with this....and I completely agree in trying to save her... but if you have tried absolutely everything you can and she is still starving to death and you decide to euthanize her- if I had to kill a snake I would *quickly*, *entirely* and *effectively* crush it's head (brain and brainstem) with something. *You would really have to do it like you meant it and not miss, is the point.* I killed a snapping turtle someone had absolutely mangled on the side of the road once and a well-aimed, heavy rock worked well....I imagine a brick would do the same. Cervical dislocation would also be humane but seems like it might be tricky with a snake.
Actually would gas work? Destroying the brain came to mind first as I don't have access to any kind gas and I'm not sure how to do that properly or if it would even work the same for snakes as it does when euthanizing rats...