At least you don’t need much documentation to get 7 days as it’s just a "Extension Denied" stamp. Your only option is a border bounce if you can’t apply for the one year extension of stay. From Bangkok and Pattaya you’ll get border bounce from 3500 baht. Personally I never would have done a 15 or 8 days overstay. You’ll be fined with up to 7500 baht anyway and you’ll get an overstay stamp that potentially could give you issues later. And if you’ll get stopped by police or anything happens within these 15 days you could face serious consequences.
Can’t give you a secure answer, but I recently got a RC myself but not in Bangkok. All the requirements under point 5 is to get registered a TM.30 on behalf of your landlord if they don’t do it themself. So if you can bring your lease contract and your TM.30 registration at this address, it’s hard to understand that this isn’t sufficient.
Personally I would have used a professional landborder company to do the bounce if it just was about getting another visa exemption. They know the name of the game and will not bring you unless they’re certain about getting you another exemption.
Three days overstay it’s not advisable, go to your local and nearest immigration and apply for a 30 days extension. You’ll be fined with 1500 baht anyway and also get a overstay stamp in your passport.
As you will be able to prove that you applied for your tourist visa while you were present and also left from Namibia, you’ll probably be able to do a border bounce and activate your tourist visa when you receive it also if you need/want more time as well.