you're required to keep the money in your account for 3 months after you get your extension, and then it can never drop below 400,000. How is one extra month of 400,000 baht a huge inconvenience?
You cannot submit them at the same time because one of the requirements for a dependent DTV is uploading the already issued DTV of the primary person.
They would need to join you when they're submitting their application as they must be in the country from when the application is submitted until the visa is issued. They don't need to be there while yours is processing.
Nowadays, it's more about opening an account where you can. People are having such difficulty, that if you can find a bank that will do it, then do it.
Did you give them your visa when you gave them your passport? You should always do that to prevent things like this. But you'll be leaving through an automated gate, you won't even see an immigration officer if you go through an airport. There won't be any issues.
You have plenty of time to do whatever you want during the afternoon. If you sleep shortly after you finish working you can go out from 2-9pm or something like that.
I got my work to let my adjust my hours a bit so I was working from 6pm-2am (6am-2pm central time) and I didn't find any problems. Especially if you can get normal weekends, though you'll never be able to go out Friday night if that's the case.