First, it is a way to confirm you are in the Netherlands and second they want to understand why you need the DTV vs. 6 month tourist visa. They might want you to log into your account and show the money. No need to get stressed. Be honest and all will go well
The DTV is less than a year old and requires one to apply outside of the country. It is very different than the type of visas one can convert to within country. I am guessing that those agents with 1000* 5 star reviews received those reviews based on retirement and work-related visas. Furthermore, each embassy has unique processes. An agent isn’t going to know the process for each embassy. I would suggest the OP to join the DTV Facebook page and look at posts related to others who have succeeded and who have failed in using sponsorship. Figure out if certain embassies seem to accept more often and sponsored by who.
probably not. They want to see you have funds that you can access and technically use while in Thailand. My understanding is you wouldn’t be able to access funds in a fixed deposit account, nullifying the rational for why they want you to have those funds