That makes sense. Too good to be true I guess. If it were just a question of not going back far enough, I guess you could request the 12 months a bit early and then do it again and put the two together to get 13-14 months of report but if immigration needs it stamped/authenticated I guess it doesn't matter. IF I can remember to ask when I go, I will see if Chiang Mai will accept that option without stamp/authentication.
I would imagine that one year is definitely needed for subsequent extensions, as the document requirements list states. They will want to make sure your funds didn't drop below 800K/400K at relevant points and thus they will need a full year to see that. So, I think you should plan for that and go to the bank early, as I will plan to do next year for my second extension. But, if I don't forget, I will update this post with my experience for others in the same situation in the future.
A couple of folks have replied saying 6 months is good for the first extension so I think I'll go that route and hope for the best. Going early enough that if it's not sufficient I have time to get it sorted with 12 months instead.
Sorry, that's a detail I forgot to mention. I used the 800K banked option. Today's bank excursion was to get a one-page statement with the current balance since no seasoning is required for the initial Non-O. I also didn't see anything about the funds needing to be shown to come from overseas, though mine were transferred that way. I guess when I apply for the one-year extension I will need a more detailed statement from the bank showing at least 2 months of seasoning and I think that might not be possible the same day but I need to research that more. I have read the same as you about the US not providing certification letters and I don't think they have done so for many years now.