There is no such thing as a 7 day entry stamp. There's not a single visa in Thailand that gives a 7 day entry stamp. You must be talking about an extension for someone who is already in Thailand, which has nothing to do with this situation.
No, you don't get a choice. If you have a visa, you use it to enter Thailand. Also it makes absolutely ZERO sense not to use it. You get 180 days EVERY TIME you enter Thailand. So even if you're only coming for a short stay, you get 180 day stamp. Then you leave and return and get 180 day stamp.
Your visa was activated the moment the embassy issued. It's valid for 5 years from that date. You're not gaining any extra time. The clock has already been ticking.
There is no way to do it without fees. The fees are either going to be tacked onto the transfer of funds, or will be built into the exchange rate. Even if you bring cash and exchange it you're going to be paying fees to the exchange place that is built into the exchange rate.
Most people use Wise to transfer to their Thai bank account, or if your bank doesn't charge extra fees you could use your ATM card and withdraw money with a 220 baht ATM fee. Just make sure you reject when the ATM asks if you want to let the ATM do the conversion for you.
If you mean proof of funds for purchasing a visa from an embassy outside of Thailand, then what bank you have shouldn't matter.
If you mean for proof of a visa or extension INSIDE of Thailand, then no, you need a THAI bank account with a THAI bank to meet proof of funds requirements. If there is a branch of this bank in Thailand it is not a consumer bank, it is a commercial bank.
Probably fine. But I'm not sure because the DTV is so new that the airlines might not know how to handle it either. If you can get on your flight you'll be fine.
Just to make sure, you got the requirements for a work permit from the Chamchuri office and not the main office at Chaeng Watthana right? Since you have a work permit you would use that office.
AIS is the largest and has the most coverage. If you're going to be living in a city, get AIS. If you're going to be living in the countryside, ask the locals what company has the strongest signal.