This is a classic example of inconsistency among embassies (and Thailand) as I have seen the Arun cooking school name pop up a few times in the past few months with now success DTV holders.
I have a joint bank account with my husband (non-Thai) when we were on an education visa. Only one person has access online and one person can scan QR code for payment, which would most likely be your gf…so would there be a point for you in this case?
They need to see that you have that amount in closing. The concept of having 500k thb is to ensure you have that amount readily available in case of emergency. Depending on where you apply, the embassy may want to see that amount in your bank account and a steady income.
you just need a thai phone number as they will send you an OTP code. You can’t change the number afterwards as you need to close the account and open a new one.