I won't say that she would not have survived-we cannot see the full extent of injuries with the feathers still there. But it would have taken intense, dedicated time, equipment and medicine so it definitely is not the wrong decision to give her mercy. Ultimately, it is up to the owner. I hear that many farmers give their chickens mercy for anything more than what some blue kote can fix; right now I have been helping someone where a raccoon basically ate half the bird's throat but the outlook is positive for the (keyword) LONG run after intensive treatment.
I think the guilt we feel, when we look at them as pets, is that we are responsible for them and ultimately that falls on us. Grieve her, but most importantly, ensure another opossum cannot do the same thing again.
She's at peace now. And that's the most important part.