Nope, you have to be working in Thailand with a work permit and paying taxes. Since you are prohibited from work for a company in Thailand, you can't get a work permit thus you cannot meet the requirements for PR.
You might want to expand on what field you are self-employed in as it could expand or even limit your options.
As for your partner, their best bet is probably looking into the new Destination Thailand Visa (DTV), which both you and your child could piggyback off once obtained, which would allow you to come to Thailand, find a job (if possible) and switch visa to a non-b.
it's 2m capital (+ 4 Thai employees) per foreign employee unless you are married and have a registered company with your wife then it's 1m capital + 2 employees, or if it has foreign majority ownership in which case it's 3m capital + 4 employees regardless of marriage status.
Arguing about the extension fee is a waste of time. Firstly, the first application for extension is about 5 months away; secondly, there is a lot of misinformation and unknowns surrounding it; Thirdly, I and many suspect there will be huge changes over the next 6 months - 1 year regarding the DTV
I've been informed by reliable immigration officers I know that they have been informed to check both finances and appointments on reentry not just when someone is applying for an extension