Is there a maximum duration to stay outside Thailand with a 1-year Non-Immigrant O Visa extension before losing it?

The answer to your last question is yes.

You just need to be back in Thailand on or before the last day of your current extension. Hopefully, several days before to have enough time to prepare necessary documents. There is no rule on how long you can be out of Thailand with a valid extension.

Theoretically, you can re-enter Thailand again on the last day (early enough) with a valid re-entry permit then go to the local immigration office straight to apply for a new extension within the working hours or queueing time if you have everything you need to apply.