Bank employees are not immigration officers. They don't know anything about this stuff. She just wanted you gone so she wouldn't have to do the mountain of paperwork needed to open an account for a foreigner.
it's still your money. But will be in your Thai bank account as long as you want to keep getting extensions that allow you to live in Thailand. Or you switch to the monthly transfers method.
The only people who can answer that question is the embassy. Each embassy sets their own requirements, so what anyone else has done doesn't matter because you're not using the same embassy as them. But many embassies are requiring much more than a 4 hour course. You can do that with a tourist visa.
Some embassies requires 1-2 weeks, some require 1 month, 3 months or 6 months. It's up to the embassy.
you cannot immediately extend. Some immigration offices allow you to apply for an extension when you have 45 days or less left, so you'd have to wait at least 15 days to apply. Some immigration offices only let you apply during your last week, so you'd have to wait at least 53 days there. It all depends on the office you'll be using.
My guess is you applied through the Thai embassy in London, which is known to be one of the fastest for processing. But there are also many other embassies that have traditionally been known to have long processing times that have been processing DTV applications quite quickly.