I understand what you mean by the snake writhin unaturally and flipping backwards. That is what I was referring to about young snakes eating. I've seen my young corns and little pueblan do that when eating for the first times.
You could be right that the snake was having a hard time but I have a feeling it was just young and inexperienced. In my case, I've seen such behavior often.
As for snakes eating their food backwards from the butt end first, that is also quite common. In fact, my colubrids seem to do so regularly about half of the time. They still get their food down fine that way, maybe it'll take a tad longer than head first.
Keeper of 5 snakes, leopard geckos, 1 green iguana, 20+ tarantulas, 2 dogs & a bunch of rats!