Better she get a Thai passport as well, otherwise she would need a visa to come back like any foreigner.
Beside, I'm sure immigration will not let a Thai national out of the country without a passport. For Thai law, she is a Thai, whatever her other nationalities are.
I've got one with my girl for house, car and food expenses (added value when she requested Schengen, US and Canadian visas) and one with my business partner to put the shop profit. I have my own account as well of course.
Yes, 600 baht for every elder. Beside civil servants, police and military got their own. Others can pay monthly to the social welfare on a voluntary basis (I for one pay 60 baht per month).