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
For what reason are you wanting to cancel it? I'm not sure you can but if you are looking for a different visa you can convert from the DTV to another visa type at an immigration office inside Thailand.
Most, if not all, consulates require you to remain in the country you apply from. I suggest you apply for the visa after you have completed your 1 weeks travel
You show invoices to your customers and receipts of money into your bank account matched against the invoices. It also helps if you show tax returns or that you are registered in the tax system in your home country. Also examples of the work you have done and a minimum of 500k baht (or equivalent) in a standard personal bank account.
Extensions are difficult to obtain and immigration don't like doing them, you will need all the information you provided originally but updated to the time you apply for the extension plus 500k baht in the bank for 1 month. Much easier just to cross a border and return
You won't need an extension. When you arrive you will be stamped in for 90 days and according to your ticket you will be inside Thailand for exactly 90 days