You are applying for an e-visa from an embassy/consulate, you must provide whatever they ask for. The 800k is for immigration in Thailand, the embassies/consulates are run by the Ministry of Foreign Affairs.
To answer the most important point, if the balance EVER drops below the required amount, it does not matter how many millions you then pour into it. You have not fulfilled the terms of getting or maintaining your extension. People regularly get refused extensions because of dipping below the required amount. Also, a joint account would require twice the funds to be shown, as immigration will assume that half are yours, and half are hers.
2 things worry me about your post, firstly, I have never heard of pr applicants getting a 6 month extension after applying, I only read that you must maintain your current unbroken extensions. Secondly, very slim chance of getting your pr in 6 months, also hurry, because the application window closes this year on 15th of May.