That's a pretty despicable comment about a woman who has suddenly become a widow.
Accusing the aviation industry of negligence in the press is pretty despicable, too. I'm sorry for her loss, but she needs to wait a year or so before drawing any conclusions on who or what might have been responsible for her loss. Right now she is emotional, and she should deal with her emotions in private rather than wailing to the press. Decisions made in haste are usually poor decisions.
(1) The obvious difference is that a car driver can easily and quickly stop the car and phone for help.
No more so than a pilot. Cardiac arrest or PE will produce unconsciousness in seconds. The driver of a car in this case will crash and die just as surely as a pilot, or more so (since there are two pilots in many aircraft).
(2) It is possible to measure the diameter of arteries non-invasively, so this condition would have been detectable with a more complete test.
Which arteries? Clots and plaques can come from anywhere, and it's not practical to look at everything, even non-invasively. Additional, things like sudden cardiac arrest may be completely unpredictable. Athletes in perfect condition occasionally drop dead from sudden cardiac death for no identifiable reason; they may have some occult cardiovascular issues, but nothing that could have been screened for in advance. And most pilots, while they are generally in good health, are not as healthy as athletes.
Overall, I think the risk of sudden incapacitation is dramatically overestimated by aviation authorities, and the methods they use to screen for it are dramatically inadequate, anyway (and only cause problems for people who may not be at risk of incapacitation at all). A more useful test might be one that checks how much sleep a pilot has been getting, but I don't think any such test exists.