If only for 3 months I would apply for another visa that can give you 3 months or come without a visa get 60 days extend at local immigration get 30 days more for 1900 bath . Then apply for dtv in home country after this vacation, to short time for 15 november

in person

u can only get visa online on the e-visa home page. closed for apllication world wide

you bring new and old passport until immigration can transfer visa to new passport number

Unless something changed the last few days it's what is = 500k bath in any account/accounts with the applicants name on statement in total. Can be 200k in 1 account and 300k in another

You need your passport

You can use statements from multiple accounts worldwide just they have your name, I used statements from my thai and Danish accounts with more than what equal 500k thaibath no problem with it's different currency

When I applied a year ago in Denmark, 1 day. Now it's 1 month statement in Denmark

How can muay thai gym say what the different embassies require, contact the different embassies yourself and ask . It's not the muay gym that approve the visa.

You don't need to leave Thailand every 6 months but atleast every 12 months, but with all the paperwork you need to extend 6 months on IO and you need live up to all requirements again when extend at IO. Much easier to use 1 day fly out and in again same day on a cheap plane or do a border run. I would anytime leave Thailand 1 day to get rid of paperwork and all the requirements you had when applied for your dtv visa.