Death most likely from a blood clot- the symptoms fit it pretty well. It is so easy, especially past the age of 50, to develop blood clots while flying long distances. That's why you wear compression socks, get up every half hour, etc. on flights.
I got a blood clot driving a truck cross country non-stop. It was pretty scary but a doc noticed it quickly.
None of this fits the conspiracy machine here, but the truth is often fairly simple.