My wife is back and out of the hospital, with follow-ups planned. No surgery done, but they couldn't satisfactorily resolve an answer to what caused her collapse, either. She was recently diagnosed with Celiac Disease, for example....and that was postulated as a possible cause, but so was appendix and several other internal body parts, and they were concerned about blood clots as well. Two days before the collapse she'd had an endoscopy, and I still suspect that the endoscopy and accompanying biopsy could have cause an infection.
Given that we were told repeatedly the primary symptoms that she experienced were indicative of hear attack or blood clots, if it's something less scary we'll be happy.
Either way, she's back now and we're all happy, albeit keeping a close eye on her. Thanks everyone for the well-wishing, we really appreciate it!