As a yardstick, three million Thai Baht either in coverage or savings is recommended to cover most eventualities. You would be looking at over 200,000 THB per annum. I take it you have already disclosed all your pre-existing conditions?
Best to take a Thai speaking person of repute with you if at all possible (schoolteachers are well respected). Bank staff like their commission so be prepared to buy Personal Accident Insurance. It is worth it particularly if you have no other form of insurance. It covers moto accidents up to age 65. The cost of an annual policy is similar to the amount you would pay an agent.
The truth! As you stated this is a rule that applies Worldwide. Checking-in online or at airport totems doesn’t avoid this issue. Airline staff operating the departure gate know how you’ve checked-in and if you have a one-way ticket. I’ve been denied boarding at the gate and also deboarded on the second leg of a three leg flight.
What I would do is get to your departure airport earlier than usual. Speak nicely to the airline desk staff and point out what has happened. You may find they can input your middle name free of charge. Someone did that but another person was charged $5 USD. Either way you will then have peace of mind Josh!