I am so sorry for your loss, it seems you got into a very tough situation but did all you could to help her and gave her and the EMTs the best chances; you're a good friend and person. I would agree with the colle ague that a PE or another clot of some sort (intracranial bleed/occlusion) comes to mind as a plausible possibility, and that waiting on the autopsy results is the best step. A serious underlying reason would also explain why the resuscitation was unsuccessful.
Where do you work? A lot of EMS protocols around the world are "No ROSC = no transport" except maybe in some extreme cases that can't be treated in the field (ie. PE in some places, or intrapartum cardiac arrest).