Can I withdraw my 800k while using the Non-O visa retirement option in Thailand?

No need to nest ANY funds once you are on an Income based ( FTT ) for 12 months option.

I just did this conversion last week at Jomtien. In my case, there was no check for the previous 800K and I just had to show the special bank statement showing my 12 deposits.

Although they didn't check me, immigration could if they want.

The requirements for the extension you just got are that you must stay at 800,000 for 3 months after your extension, and it cannot drop below 400,000 the rest of the year.

That is a completely separate requirement from what you are trying to do with the transfers.

Next year when you go to apply for your extension based on monthly transfers, they will request you to provide 12 months of bank statements. They will use these statements for 2 things.

First: They will verify that you met the requirements for last year, which is the 800,000 for 3 months after you received your extension and that your bank account never dropped below 400,000.

Second: they will check if you received the proper international transfers every month for the previous 12 months.

If either of these is not the case, you will be denied your next extension for failure to meet the requirements.