Can I spend part of my 800k Baht requirement for a Non-Immigrant O visa and still comply with the rules?

The 800,000 baht must remain in your account 3 months after you receive your yearly extension and also 2 months before you apply for your next yearly extension. You are correct the balance cannot drop below 400,000 baht at anytime throughout the year, so basically 5 months must be maintained at 800,000 and 7 months you can access the funds if you wish.

The requirement is that you must have 800,000 baht in the bank 2 months before you apply (not 2 months before your extension ends, it needs to already be in there for 2 months from when you apply for your extension) and then must remain for 3 months after you receive your extension. So that's about 5 months that it needs to be at 800,000. Then it can not drop below 400,000 the rest of the time.

They will check you followed this next year when they require a 1 year bank statement from you to look at the 3 months after your previous extension plus the remaining months.