Cigna Global will take into account whether your pre-existing conditions could be regarded as 'resolved' or 'controlled'. Most other companies will simply exclude them.
I have flown regularly with Air Asia, but only internally within Thailand. I don’t know if they would ask you or not. However, you should have your DTV to show on your phone and also printed out. That would satisfy any requirement for ‘some type of visa’.
Depends upon the airline. Checking-in online or at airport totems does not avoid this requirement (if applicable). Boarding gate staff know how you’ve checked-in and can (and do) stop you at the gate and ask for sight of; return/onward ticket or, ‘some sort of visa’.
90 day periods are not cumulative. Your next report is due 90 days from the last. Immigration officers at both airports and land borders are not concerned with 90 day reporting.
Depends how far you are from a border. Then there is a need for a visa for the other country. Incidentals such as food and drink during your journey. If you have to stop overnight a hotel room is required. Time away from home that you could be doing something useful or even productive.
Haven’t been to Jomtien immigration but can appreciate it must be busier than many other immigration offices. My last 90 day report was in-person for same reason. The girl on the form checking desk filled out a TM 47 for me as she wasn’t busy. Very good service IMHO.