My second Non-O extension based on marriage is coming up. I have the necessary 400,000 in the bank. 

But I can't find a guidance that tells if the entire amount must be proved to have come via foreign bank transfer from abroad. Or if some of it can come from a Thai salary, earned while on a legitimate work permit. 

For my first extension in 2021, the complete 400,000 came from abroad. Over the next few months, I got an official Thai salary. So the monthly salary added Thai money to the bank account and I withdrew cash for living expenses. The account balance always stayed above 400,000. The bank book shows "salary" as the origin of incoming payments after getting my first extension and work permit.

Would an IO consider the "mix" problematic: "Your foreign cash is now only 300,000 and your Thai-origin cash is 100,000. Because our mission is to make life as difficult as possible for you, we will assume that you used up your foreign cash first, before refilling your account with your Thai salary. But Thai-origin cash does not meet the requirements for the 400,000. Extension denied." 

Or is the origin of the 400,000 irrelevant for second and later extensions based on marriage?

If I ever switch to the monthly method for future marriage extensions: can the monthly income of 40,000 originate from an official Thai salary? 

There is no requirement to prove the 400k baht came from abroad for the extension of stay. 

With the way petrol is going up 46 baht per litre for 95, I would suggest you budget for 500,000 very soon, and nobody gives a toss where it comes from.

hknn, with the 400,000 system you must have that money in a Thai bank in your name only and there is no requirement to show it has come from overseas. It is only the income system of 40,000 per month that requires the money to be transferred from overseas into a Thai bank account in your name only