Is there a grace period for late extensions on a Non-O visa in Thailand?

there is NO such thing as a grace period. At most offices you can apply for an extension from up to 30 days before your current stamp expires.

IF you apply AFTER your current stamp expires you're on overstay and pay 500 baht a day

I don't think there are any circumstances where you can be late. You have 45 days before the current extension or visa expires to start the process. If you're past the validity date of the visa or past the date of your extension you are in the country illegally.