I understand 4 seater production is safe, although there will be a gap in production until the isofix issue is resolved. Must say, biased as I am in that my first Morgan was a 4 seater, that it would be a very sad day if they were to disappear.
I had lap belts fitted in my 1981 car as rear belts weren't fitted as standard and my two sons didn't come to any harm, although they did catch quite a few flying insects as their heads were above the height of the windscreen. Happy days.