The accelerometers in consumer grade drones are not nearly accurate enough for inertial navigation over any significant distance.

You don't need to fly the entire route under INS. What it will do is keep the UAV from dropping out of the sky in the presence of jamming.

Satellite navigation is often jammed in war.

To the extent of limiting reception and degrading accuracy, sure. But if all you need is the thing to have a general idea of where it is in the presence of jamming, degraded accuracy is fine.

The US military can degrade the accuracy of civilian GPS signals without affecting the encrypted military signals.

That was done via just not broadcasting some information to civilians, not jamming.

Anyone who runs a satellite positioning system can refuse to broadcast position data, but there are a bunch of global systems up there. The US has one, Russia has one, the EU has one, China has one, and other countries have regional ones. If you want to have them not broadcast data to deny someone position data, you gotta have all of the US, Russia, the EU, and China on-side. And if that's the case, you've probably already won the conflict anyway.

Frequency hopping can work around some jamming, but a powerful enough jammer will overload and desensitize the receiver making it unable to hear anything.

It's still proposing covering the radio spectrum as a whole. Traveling further along the same if you have a powerful enough transmitter that's directional enough, you can just burn the drone's electronics out. But nobody can actually do that.

Optical based navigation would be immune to RF jamming, but that's not going to be found in consumer grade drones.

Yeah, and it's going to require line of sight or a relay, like that second drone, and won't work in all weather.