Can I extend my Non-O Visa if my bank balance was briefly below the required amount?

If I was a betting man I'd wager they're NOT gonna let you get the year extension :(

Unfortunately the rules are totally specific and sadly it's not keep as close to 500K baht in the bank as you can for 3 months before you apply for your extension :(

You said you have 2 months of the money being at 500K baht meaning you're short 15 days or so. They MIGHT let you run on overstay those 15 days until your money is seasoned the 3 months required and have you pay 7500 in overstay fines.

OR

They might tell you to leave the country, get a NEW Non-O visa and start the process over once you come back <- which might not be a bad thing because it'd be the FIRST yearly extension on that visa and you'd only need the money in the account for 30 days

Good Luck, only your immigration office can tell you for sure and I'd suggest you go out there way ahead of showing up to apply for your extension to find out what they can do or will do for you..

How long has it been since the balance returned above 500k?

Only Immigration can tell, but from my experience they are hard core.

you heard correct: (5) In the case of parents, funds must be deposited in a bank in Thailand, under the father’s or the

mother’s name, of no less than Baht 500,000 for the past three months.