this is the current police order. Check under 2.2 it says inland travel doesn't require you to file a new TM30 after return at home. Only if you come from abroad. . . . . . . . . . . .
you would not have been able to register an address under the TM30, without the departure card. Because in order to file a TM30 correctly, the hotel or the landlord need the departure card number. If you lost the deparure card but have been told by whomever, that your address and TM30 has been registered successfully, this cannot be true
normally if your wife vouches for you, uses her phone and address contact details for the bank and you bring a certificate of residence from immigration along, most bank branches will accept opening a savings account for you. If one branch says no, go to another bank.
You can get the 90-days Non-Imm-O Visa based on having a half Thai child, by showing the birth certificate with your name on it, when applying on the Thai embassy or consulate.
However for the application for a “1-year extension of stay based on raising a half Thai child”, you will need the parential rights approved by a Thai family court. And this process might take some months to complete.
You could set the approval process into motion before you go back to the UK by June, and when you return, you can either return with a 90-days Non-Imm-O again, but also on a 60-days Tourist Visa, and from there do the extension or, for the later, the conversion to a 90-days Non-O. Money 400K in a Thai Bank account is required, and you said you are complying with.
So actually everything is a question of correct timing
you can convert the 60+30 days tourist visa to a 90-days Non-Imm-O based on being over 50, at most (not every one) immigration offices. If you find it unable to open a bank account on a tourist visa, this might be a hindrance, as the Immigration might want to see the proof of a certain sum of money having been transferred from abroad, to be able to apply for the conversion. A catch-22 situation. You would fare much better if you applied for the Non-Imm-O/A in the UK. This will allow you amble time to fulfill the financial requirements