Cannot switch passports at land crossing must fly internationally. On returning to Thailand use their Thai passport. A relation of my wife was initially refused even at airport and said she would have to use her German passport. She asked to see a supervisor who immediately agreed that she could use her Thai passport.

I've already got that in the bank so could I convert this multiple entry visa to a normal Non-O before the first 90 day mark. Was my mistake to select Multiple entry on the visa webpage?

A 12 month period of transfer may be needed. Also if you miss a month, say your pensions during the Xmas period give you Jan's money in Dec which means you will have missed a month Jan and have to start the 12 month period again. Also some immigration offices may only accept the 400k deposit for at least two or possibly three months. The bank account cannot be in joint names. Last point if you use one of the fast transfer methods such TransferWise the funds appear as coming internally from Bangkok so you will need extra paperwork proving the money came from abroad.

Thanks for the heads up. Will be doing this next year sometime but did not realise the full extent.