Sadly
@Da***
sounds to me like that lawyer is not all that clued in on extensions of stays. :/
There is no proof required that the 800K baht originate from abroad when applying for the yearly extension from a Non-O visa. You could certainly transfer the money from the joint account to one in your name only without any issue and as long as it's seasoned the 2 months before you apply for the extension you should be fine.
Now as far as whether your office will accept a joint account with double the required amount, that is between you and them. As I said, some will and some won't