The requirement is a minimum of 65.000 Thb a month.
If you reach that amount is for you to see, best is to take your bank book and passport and ask at the Immigration Office if your monthly transfers meet the requirement.
Cost of every application of Extension of Stay is 1.900 Thb, it does not matter if you get 30 days or 2 years or even when the application is not approved.
If you like to use the 800.000 Thb banked money, it must be in a Thai bank at least 2 months prior application date and at least 3 months after apllication date and the rest of the year not less than 400.000 Thb. Your promise will be checked at next year application.
She has 6 days overstay, she will be fined 500 Thb a day. The officer may or may not grant her a new Extension of Stay based on being unablte to travel due to covid, but the officer can also tell her to leave the country and give here 7 days to organize it.
When people are sick, action has to be taken BEFORE they go on overstay.
I do not see why you should change into a Non Immigrant O-A (Long Stay) visa based on being over 50 years of age with total different requirements and a healt care insurance.
Okay, The 400.000 Thb in a THAI bank or the 40.000 Thb monthly income in a Thai bank is a requirement for the Extension of Stay if you already have a visa and entered Thailad using that visa