"Is S Korea allowing in more flights now - and if so, with mandatory health checks and isolation periods ?"
There have always been flights coming in, not sure how the numbers have changed. Everyone is subject to a 2 week quarantine period at a cost of about $1000US if you don't have a permanent address. Otherwise you just quarantine at home. They put an app on your phone to track your movements and check you're not going out.
"Are there any estimates for what % of the population is immune ?
WIth so few cases, it must be very few - are there fears of^ a 2nd wave ?"
No idea about immunity.
Fears of a second wave - some people are very blase. Some are still in panic mode. Korea is a very group based culture so when people see others are going out, they tend to do the same rather than listen to government advice, which is a worry. I would say the government have greater control than in the UK, perhaps so if it does come to lockdown situation, I think it would be pretty strict.
Much better medical facilities though which has helped a lot.