Immigration Supanburi just confirmed to a german and his Thai wife that he can convert using TM2 from 2018 Non-Imm-O/A to a Non-Imm-O based on marriage. Husband has however to meet the financial requirement at the date of the conversion. So actually in SOME immigrations they are willing to do it

The whole duration of stay (which is one year, regardless how long you intend to stay) if you only want to stay for three months, then this is not your ideal choice of visa type. For three months, a single entry tourist visa would be sufficient. And in case you wish to stay longer, you can wring 6 to 9 months out of a Multi Entry Tourist Visa

you can apply for Non-Imm-O based on marriage on almost every embassy or consulate of Thailand in any country (however the financial proof, some want to see and others not) and also you can do a conversion from a Tourist Visa to a Non-Imm-O based on marriage inside Thailand on some Immigration offices

the money does not need to be in an account in Thailand when you apply for a Non-Imm-O single entry in your country. You will receive a 90-Day Stay permit upon entering Thailand, time enough to open the account and deposit the 400K for the two months-period needed prior to your application for Extension of Stay Based on Marriage

no, it needs to be an account in solely your name. If you and your wife keep a shared account, the money will have to be 2 times 400K = 800,000

no, no other solutions than those two explained by Robert Lagas. You will have to follow Thai rules, not your own rules. If you have the money, just deposit the 400K in a thai bank, it is the easiest method to obtain a Non-ImmO based on marriage Extension. And the money is a good emergency fund in case you need hospital

not very clear at this moment. We don't know

Bernd Moonfruit this is not about a heatlh insurance for vistors to Germany, but for health insurance for applicants to a Non-Imm-O/A in their home country, forvan extended stay in Thailand. In case your German insurance company was willing to sign the "foreign insurance certificate", and the Embassy or Consulate will accept it, you re fine. At this very moment, most insurances refuse to sign the "certificate", because it is out of their jurisdiction. As people keep explaining their experiences, it is at this very moment not clear if your querry for an extension without having a THAI insurance has any chance. We are awaiting more telltale experiences from people who are affected by the change

it is possible as you have it planned. Make sure when re-entering Thailand for the 30-day exempt, to have a hotel booking for the first few days and an out of country ticket within the 30 days (sometimes an outward ticket within the 60 days is accepted as they know that you can extend the 30 day exempt for 30 more days) and have CASH minimum of 20,000 Baht or the equivalent in another currency ready to show. Preferably you should do the re-entry for the visa exempt by a land border, Laos to Nong Khai is known for leniency and easiness

most likely the documents needed are: Application form T.M. 7

1 photograph (4x6cm)

Copy of passport, especially Non-Immigrant visa and TM6 arrival/departure card

Fee 1900 THB

Copy of your marriage certificate (if married in another country, a letter of evidence is required from your embassy as well)

Copy of Kor Ror 2 (marriage registration or long tabian) from amphoe office, dated not older than 6 months from issue of copy

Children’s birth certificates (if applicable)

Copy of spouse’s house registration ("tabian baan")

Copy of spouse’s ID card. When you use the deposit method, you will also need your bank's "letter of guarantee" that you had 400,000+ in your account for the last 2 months. you will most probably been given a waiting period of ONE MONTH, during which your application will be sent to the head office for confirmation. During this spell, some immigration officers will visit you and your wife at your home, making sure you live together. I also advise you to buy one single re-entry permit (1000 THB) in case you have to travel abroad, so your EOS does not become invalid upon re-entry to Thailand