The problem is with the way GPS works. Your device gets telemetry from the satellites. A fake signal can screw up the whole system.

Yes Galileo supports encryption. But as far as I know it's not in use. Has been trialled only. But I know all Airbus aircraft only support GPS satellites and nothing else (yet). I assume Boeing, being American would be the same then.

As far as solutions go, an aircraft can navigate fine without GPS. It can update its position from ground navigation aids and if they are not available it can still Dead Reckon very well. The navigation error very slowly grows until it's out of the black spot and can use GPS or navigation aid to increase its accuracy. But this navigation error on the time frame of say an hour is a matter of kilometers at most, not dozens.

Nope. And more importantly, it looks like nobody considered what might happen if the signal gets spoofed. The backup systems that are supposed to keep working if GPS breaks also break due to these spoofed signals.

GPS is old, the amount of data you get from the satellite is small, essentially satellite id and timestamp. If we would redesign this today, you could include a digital signature.

Sure, but... you can google this to verify ... one can probably manipulate GPS by introducing delay, i.e. resend data from a sat that was hear some seconds ago. With this signal the location will be off.