You must apply using the Thai consulate that serves your current location. Some consulates restrict applications to residents, so if you are not you would have to travel elsewhere to apply. Once you've applied you need to remain in that location until the visa process completes
You should be able to convert your DTV to a Non-B at an immigration office inside Thailand with the correct paperwork from the company that will employ you.
You apply for a residence certificate at the immigration office where you have a TM30 filed. If you are staying at a hotel they should do that for you otherwise your landlord or the owner of your accommodation should do it
Freetrade won't work, you need the funds in a standard bank account. You should also show your earnings via invoices matched up with incoming funds in your bank to prove your income. And if you are registered to pay tax or have tax returns include that