Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that she had several chronic health issues (recurrent, metastatic, mammary cancer, kidney disease) but I am sorry to hear that your kitty passed away so suddenly and dramatically. It is perfectly normal to ask why and how this happened. It is part of the grieving and healing process to try to understand.
A severe, sudden allergic reaction is one possible cause of sudden respiratory distress and death. Animals with such a severe reaction will often have hives, facial swelling, swelling of the tissues around and in the pharynx, vomit, can have diarrhea and can collapse. This would be highly unusual in her case because you don't report any hives or facial swelling, but it is possible.
The most common cause of sudden death in cats is collapse due to throwing a clot from undiagnosed hypertension due to primary heart disease, usually cardiomyopathy, or kidney disease.
Cardiomyopathy is a primary heart muscle disease which can be a silent killer. Cats with cardiomyopathy can appear to be normal, they will compensate for their heart disease for a period of time and when failure occurs it can be very sudden. In retrospect owners may report that their cat was slightly less active but that they assumed that was an age related sign rather then a sign of heart disease. Radiographs of the heart may look completely normal. The only good way to diagnose the disease is with an ultrasound of the heart, called an echocardiogram.
Kidney disease can also lead to hypertension and blood clots. This seems more likely with her because we know that she had kidney disease. It is possible that she had previous small blood clots thrown given her previous history of open mouth breathing. This time around I suspect that she suffered a saddle thrombus clot (a blood clot which blocks blood flow to one or both back legs) which would explain her rear leg weakness, as well as a possible clot to her brain or lungs which would explain her final collapse and vocalization and then death. Cats with cancer are known to have a much higher incidence of blood clots too. Cancer is an inflammatory process which predisposes to blood clots.
Another less likely for her cause of sudden difficulty breathing due to pulmonary edema is biting an electrical cord. Those cats should have signs of a burn on the gums or tongue. This is much more common in kittens then adult cats.
Other rare cases of sudden pulmonary edema and difficulty breathing are pancreatitis, systemic bacterial infections or cancer (due to blood clots and metastatic tumors in the lungs themselves).
A previously undiagnosed, ruptured, bleeding tumor in her abdomen could also cause the symptoms that you saw. We see this more in dogs then in cats, but it is possible as a sudden drop in blood pressure could explain weakness and sudden death.
Please don't think that you did anything to harm your girl, or that fluids a few minutes earlier would have changed what happened.
I believe that she probably had undiagnosed hypertension and her cancer led to an increased probability of forming dangerous blood clots and sudden death. It is horrible for you as you had no time to process her being sick as sick as she was, let alone her death, but please take comfort in knowing that she didn't suffer long after the blood clots and it sound like she enjoyed a wonderful life with you doing the best that you could for her until the end.
Creatinine kinase is a muscle enzyme which increases with the breakdown of muscle tissues. That increase too is a likely indicator of a blood clot that caused muscle damage and enzyme leakage.
Please let me know if you have any further questions.