You need to remain in the country you apply from until the process completes. They can call you in for interview or ask to see every page in your passport to check you remain there. No being there means your application can be rejected
For the initial 90 day Non-O inside Thailand most immigration offices want to see an international transfer so bank transfer. For an extension it isn't a requirement but for that amount of money you need to declare it on entry
I don't believe a DTV can be cancelled but it can be converted to another visa type at your local immigration office. Whether or not your school will provide the paperwork for that is between you and the school. I don't believe you can get the Non-B at a consulate outside of Thailand as you can't have 2 current visas
You would show proof of your business, forget IG, business registration, proof of taxes, some invoices correlates with your business bank account, etc. Plus a letter from your business giving you permission to work remotely from Thailand. You apply and once your DTV has been issued your wife applies quoting your DTV and providing marriage certificate etc