You can apply for the non O if you are already in Thailand, but the problem is you are unlikely to be able to open a bank account on a tourist visa (do you have a bank account already?).
lovely photos, you look like a nice couple. I understand your dilemma. I think that is the best solution for now. I can see why she would not want to move as she has a good job, healthcare and pension if she stays here. Have you thought about asking your medical people if there is any way that you could be tested in Thailand once a year instead? The medical facilities here are excellent and that way you could spend 6 months each year here.
No. Opening a bank account is pretty much impossible now in Thailand unless you have a long term visa. Get the non O visa before coming to Thailand and Opening a bank account should be no problem.
From what you have stated, I think you might have a very difficult time providing sufficient evidence. Would it not make more sense for you to visit her in Thailand until you get married, as it's easy for you. Remember agents cannot help you by bribing someone for this process. So if you are going to go through with it, you need to get her evidence organised and indexed, in the same order immigration ask for it, so that the people making the decision have no excuse to deny her. Good luck.
the basic process is:1. Get online non O visa from the Thai Embassy in the uk. 2. Travel to Thailand ensuring immigration stamp you in with the visa. 3. Open a bank account immediately and deposit 800,000 baht.4 Go to immigration and get a list of requirements to extend your stay for 12 months. 5. Wait until your money has been in the Thai bank for two months.6. Take all the documents on the immigration list and apply to extend your stay. It is one a4 form with about 15 questions to complete,with simple questions like name and date of arrival in Thailand. Pay 1,900 baht, wait a few minutes and you have a visa for a year.
make it easier for yourself by getting the non O visa before you come to Thailand as others have stated. It used to be possible to do it the way you planned, but the banks are being very strict about opening accounts on tourist visas, because of all the high profile investigations into mule accounts. A few minutes doing your visa online before you come should make the process smooth, with no need for agents.