Good and clear comment, but he still wanted a Non-O visa based on marriage to a Thai national. He just got a bit unsure about the financial requirements and thought it required both income and a lump sum.
It’s just your friend Stuart who’s got things a bit mixed up and is talking about an extension of stay in Thailand.
You can apply for a Non-O, as you’re trying to do, by providing documentation of at least three months of income of a minimum of 40,000 baht per month.
400,000 baht is just an alternative basis for applying if you’re not using income.
He’s not applying for a Non O visa inside Thailand and can apply for a Non O e-visa through the Thai embassy of London only with three months income.
You cannot apply for two visas at the same time if the application isn’t approved or denied before you travel. If you hold a passport from the list of 93 nations eligible for a 60-day visa-exempt stay you can first enter Thailand under the visa exemption and activate your visa later by a border run once it has been issued.
If your visa application is denied, the alternative is to use land transport companies to make border crossings in order to obtain new visa exemptions.
If you still obtain an income affidavit from your embassy, you only need to prove a monthly income of at least 40,000 baht — without a lump sum of 400,000 baht in a Thai bank. For retirement it’s at least 65,000 baht a month.
To apply for an METV, you must formally document your need and provide at least two entry and two exit tickets. Enforcement of this requirement varies between embassies and consulates: some require two entries and two exits, while others issue an METV with only a single entry ticket. For a precise answer, it is recommended to contact the relevant embassy to confirm their specific requirements.
Alternatively, you may also choose to stay for 90 days by applying for a 30 days local extension before each departure.
Regarding bus or train tickets, most consulates/airlines would probably accept them as long as it is clear that the ticket is for a journey that crosses a border and the final destination is in the neighboring country.