What should my Mom do if her insurance company won't provide the required certificate for her retirement visa application?

Yes, you can normally apply early—often 30 days, and some offices allow up to 45. The new extension should run from the expiry of your current permission, so you don’t lose those days. Best to confirm the early-application window with your local immigration office. Official Immigration guidance can vary by office; a 2026 Immigration public handbook states applications may be submitted up to 45 days before expiry.

We do ours in Jomtien 40 days prior. No problem.

a new 1-year extension starts on the day after your current extension expires.

At most offices you can apply for a new yearly extension when you have 30 days or less left on your stamp, and at some offices Bangkok, Chiang Mai, Samut Prakan, Jomtien, Phuket) you can apply when you have 45 days or less left on your stamp.

You don't lose any time applying early because the year extension is added to your current extension expiration date

Check with your office to see if it's 30 days or 45 days before that you can apply and go from there.