3rd only remitted money to Thailand is taxable in Thailand , not earned globally .. and 4th to take the double taxation treaty into account (if in place)
Much easier to do it in home country. I tried to do it in Thailand but was so cumbersome (without an agent) that i have done it in home country instead and not used agent for in country extensions either.
Easiest to apply for non-o through local embassy. The London embassy certainly does not require a health insurance and neither does the extension in BKK. Non-OA however does require a health insurance as does the Non-OX i believe.