While for me the Non-O based on being over 50 is superior if you're going to be doing remote work from inside Thailand you should get the DTV visa, that's what it's designed for. If you don't think you currently qualify using the workation option use soft power
it will depend. First you need to spend 180 days or more in a tax year inside Thailand then if you bring income into Thailand you might have to pay tax on it depending on your personal situation and whether the source of your income has a tax agreement with Thailand
you'll need to scan the documents, potentially combine multiple documents into single pdf files if you need to upload more than one document in any one section etc. I'm guessing the person that gave you the instructions has not done this since all of the Thai consulates moved to the evisa online system
Thanks. Rules out my theory that chaengwattana may just be getting fed up of folk that don't bank there asking for the letter rather than at their own branch