yes you can in theory. The immigration officer in the airport may just wave you through or could question you about staying in thailand on visa exempt and possibly deny you entry. If you haven't stayed in thailand for more than the 1 visit and extension then it's more likely you'll be ok. I would suggest you make sure you have the entry requirements with you in case they ask on your return. These are cash in your pocket I think
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baht, a place to stay and an onward ticket within the 60 days of the new visa exempt. Unfortunately in thailand it's never a black and white answer it's always a depends on the IO even though the rules If you call them that dont actually change. I hope this helps
You can't have a non o visa as its valid for 90 days? And you can't extend it for 30 days either. It sounds like you entered on visa exempt i.e. no visa at all for 60 days and then extended at immigration for 30days. If you did enter on visa exempt then if you leave the country and then reenter then you should get another 60 days visa exempt but that of course depends on how many times yiu have done this previously and whether immigration will stamp you back in.
All I can say is just enjoy your trip you have all the correct visas for your stay, in future my advice is to not beleive here say or Google as most are out of date or incorrect. Pop any questions on here and you get great advice from very helpful and knowledgeable people.
you will only get a multi entry visa if you can demonstrate you need one. So when applying on the e visa system you will need to show flights or transport out of thailand and then back in again, just arriving with no exit will not get you the multi entry visa
You need to contact a muay thai gym first and ask them about their course and if it is DTV approved. Yes you pay yhem for the course before you apply for the visa since the muay thai gym will need a copy of your passport to send details to the ministry to get you registered and give the muay thai gym the appropriate paperwork to send to you, you need this as proof in your application to the embassy.