Availability of other national broadcasters

You could get the 819 lines transmissions on a 405 line telly, but as two pictures side by side as the line rate was almost exactly twice No audio, because the transmission channel bandwidth was about 11 MHz I think, so the audio sub carrier will have been way higher than the UK 405 line one which was at 3.5 MHz. That's what killed 819 lines, way too much bandwidth required.

Could they possibly have got audio by tuning to the right frequency? Though then you wouldn't have got the picture instead.

Once I bought a cheap tiny portable that ended up being made to the European standards, so you couldn't get the picture and the audio clear at the same time, as it was working to the PAL B/G (or was it D/K? I can't remember now) offset rather than the PAL I one. So you either had a slightly fuzzy picture but good audio, or a good picture but fuzzy audio, or some balance between the two.

That might have been possible, some 405 line TVs didn't employ inter-carrier audio reception where (without getting very technical) the entire composite video and audio signal is demodulated, with the audio demoded off the back of that. In that situation it's not possible to tune into the sound carrier without the vision carrier being present (or being in the 'right' place). However early 405 line tellies had separate full demod for vision and sound, and I think that was exploited in the 60s by a couple of pirate 'radio' stations that came on after close down of the TV channels, operating in sound only !

I have a question that has probably never been asked before...on the same token can French TV channels in Southern Corsica be seen in Northern Sardinia and viceversa given that the two islands are only 7 miles apart?

Probably can, (if you are using a receiver than complies with the other country's standard). Nothing very remarkable about it though, the same will apply along and near any national border

I understand, thank you!