Actually that's might be up to the airline. I've managed to cancel the first leg and allowed to take only the second leg, one time. But this was 20 years ago.
But it is correct very if I hadn't turn up at all for the first leg and not canceled it it would have canceled my second leg.
You can arrive on exempt and 15 days before end of it apply to transfer to non-o 90 day visa (for retirement). I think you need to show (longer term) rental contract for apartment not just stay in guesthouse). For the non-O visa you need to have transferred 800k/etc to Thai bank and have proofs. Then, once the money has sat for 2 months you can apply for 1 year extension of stay for retirement reasons.
To exit thailand and cone back and not lose the stay you need to get reentry permit. You can search Google for that.
Insurance is not required for O visa.
Just for non-o just for entry into country (TP) etc.
It may be possible to apply for non-o in your country but I don't know the procedure.
I tried to find this on the TATs website but it's impossible. You'd think they would be able to gather the information updated in one place instead of people reposting infographics which may be months old...
But nobody cares about making things easier right?