From what I understand, only non B visa (work visa) will have custom exempt. There is Samsung factory here so replacing your TV and fridge is very cheap.
I'm registered with my country's embassy in Thailand so they know I am here. In case of emergencies that needs evacuation like war etc or death, they will contact my immediate family back home. If your home embassy has such service, it makes sense to do so.
Did you have proof of revenue and income to show them? Just having an online business doesn't prove income. Present your invoices and revenue stream and tax documents maybe?
The soft power activity must be Thai specific and the school or gym must be approved for DTV application. Don’t think Badminton is a Thai sport. So maybe that’s why you were rejected?
It looks like they have no business but they are probably one of the sellers on Shopee or Lazada. The small shops selling bulbs and electrical stuff on my soi does their business online too.
I understand they do that with marriage visas and will even interview neighbours of yours. It's their job to make sure you're here on legitimate reasons. You can kick a fuss and have them revoke your status and that would do you and your family no good. You can read on this group how many dumb people try to beat the system, find loopholes etc and that is why they check. If you want to get angry, do it at the morons who screw it up for the rest of us.