I will try to break up your questions and sprinkle in my answers. Certainly, I can be wrong, but here goes:
Enemy action against particular satellites.
Not particularly worrisome from a navigational standpoint. Unless the entire constellation of satellites is removed, this is merely a degradation in GPS navigation. I believe there are 30 in orbit. Only a handful of satellites are required to get a fix.
What the attacking country should be worried about is the assured counter-attack, as the US has a publicly stated policy, re:attacks on our satellites is unacceptable.
Enemy action (hacking) of command and control systems.
Only in the movies. It's possible but highly unlikely. Please do not think of military systems as Mickey Mouse Club toys.
Deliberate administrative decisions to degrade or shut down service, possibly on a regional basis, by system operators.
This happens in military theaters, right now. The satellites aren't shut down, but attempts are made to jam the signal, by all parties including the US. The military channel cannot be spoofed, but the L1 civilian channel certainly can. Once removed from the locality of the jammer, normal activity resumes.
As to the ability to degrade or shut down service, the EU and Russian systems serve as an interesting check against the US system. As recently discussed, event if Selective Availabilty was to return, the other satellite based systems will soon expose it.
I don't think these outcomes are likely -- I'm not a kook and I don't buy lottery tickets. I just think that the likelihood of these outcomes is difficult to assess and that the prudent mariner should be prepared.
The prudent navigator should have multiple GPS receivers that are battery powered. They should be unconnected to the ships power or chart plotter. This, in the far more likely event of catastrophic ship power failure. Simply put, single sourcing your reception of the GPS signal, and therefore it's possible single point of failure, has a much higher probability of outcome than systemic satellite failure. Having battery GPS receivers means that you can continue your journey.
The backup to GPS is another GPS. Systemic (total) failure of the GPS satellite system is a low probability event. Celestial navigation isn't a practical backup to GPS navigation, full stop.
It is not emergency, abandon ship navigation, either. Liferafts aren't navigated anywhere anymore. The preGPS advice given by Dutton was to bring all the CN equipment with you. That has all been completely superseded by EPIRB & PLB beacons, bringing SAR right to you. Add in an AIS SART beacon, plus a waterproof VHF (+DSC) for broadband local alert. Toss on a Radar SART for good measure. The sextant might have represented safety, preGPS, but I'd much rather have the modern equipment. Especially since I'm not "navigating" the liferaft, I'm simply adrift.
PS. I am not anti-CN. However, I have listened to, and participated in, far too many fanciful conversations about how someday, it will save your butt. That's just wishful thinking. Modern equipment has completely subsumed CN. It's well done and dusted. Would you rather jump into a liferaft with a sextant or an EPIRB? No contest. Would you rather have a sextant or another GPS receiver in the event of ship electronics failure? No contest.
Should we stop exercising our minds and discovering how CN works? Again, no contest. There is something magical in the pursuit.