What should I know about renewing a Non-OA retirement visa with an expiring US passport in Thailand?

on Page 49 of the latest, publicly available update of one of Thai Immigration Bureau's Field Manuals/Training Manuals titled: " สตม imm 108 ",

it has been long–standing practice in the real world at the various, nearly 86 local Thai Immigration Offices across Thailand, that Extensions of Stay, even Under Consideration (U/C) Periods for Extensions, can be granted for less than the fully permitted time period, up to the maximum validity of any passport

in สตม imm 108, the Thai Immigration Bureau gives an example, sample scenario of:

Applying for an Extension on 05 June 2008, but their passport expires on 01 August 2008, then the local Thai Immigration Office is permitted to grant an Extension up to 01 August 2008

More importantly, the Thai Immigration Bureau cites:

Article/Section 12(1) of the Thai Immigration Act B.E. 2522 (1979) as their legal authority to do so or otherwise within their discretionary authority

There's no requirement to have ANY amount of time on your passport to apply for a 1-year extension (not a renewal). If you don't have enough time on your passport for your extension, you'll just receive a short extension that's only valid until the expiration date of your passport.

The 18 months part is a fairy tale.

As long as you have one year or more time remaining on your current passport, Thai Immigration Officers will grant your request for an extension up to and including the expiration date of your current passport.