How can there be a cloud cover at FL300 so large that the plane cant navigate out of it?

Lots of scenarios where the pilot isn’t allowed to diverge from a flight path without getting into how freak events happen with the weather all the time.

Cirrocumulus clouds are common on that flight path and can extend up to FL450. Is it rare that they are that high, for sure, but can it happen, definitely.

Cirrostratus clouds are also a possibility and they often extend up to FL400. Whenever the sky has a soft white haze, these are a common cause.

Will either of those guaranteed ruin an eclipse, probably not. But still saying guaranteed no cloud cover obstructing the view is very much inaccurate imho.

Yeah when normal people talk (not pilots) an event with >99 percent probability should be deemed a certainty. As a pilot however i agree with you that you absolutely cannot make any assumptions about cloud cover

Depends on how you define normal people. If you are someone who is ponying up $25k+ for this flight on secondary markets you also should know that the chance is non-zero, albeit close.

Yeah thats fair also.