We decided to do a necropsy because I wanted to know why she took such a sudden downturn when she was eating well, etc. Cancer? Infection? Aspiration of something? Defect?
Supposedly her heart and lungs did not look normal. They sent the tissue for biopsy and the vet will tell us more once the results are back. So we are closer to having an answer.
Half of me is relieved to find out it looks like she just had a bad heart and lungs and there was nothing we could have done, and half of me is devastated that I didn’t know going into the surgery. I never would have done it.
Nothing can bring her back, but I have learned that, for me, I want to a) perform preventative spays as soon as I get future guinea pigs, while they are young enough that things like lung and heart disease haven't had time to develop and b) push for more tests before any surgery (chest X-ray, perhaps) so I can make a more informed decision about the risks of that vs. palliative care. Maybe these things wouldn't have made a difference for her - my gut always told me she was fragile and that's why we didn't spay her when she was young. It's impossible to say.
Anyway, thank you all for responding to my thread and helping me process this roller coaster of health issues with my pig. I miss her very much.