the one year extension is ADDED to the expiration date of the 90 day stamp from the Non-O <- which means with the Non-O and the year extension you have 15 months total 🙂 and each year after that will be added to that same expiration date year after year
This is a general list of things required to get a 30 day extension on a 60 day visa exempt or 60 day tourist visa entry stamp. Remember there are over 80 offices in the country so what yours requires might be a bit different
More and more offices are requiring that you have a TM30 receipt (showing you're registered as staying in the area served by the immigration office)
At this time people from 90+ countries (Canada included) can get in to thailand free stamp (visa exempt) entry for 60 days without purchasing any visa.
There is talk by the thai government that they will reduce the 60 days down to 30 but even if they do that between now and your planned trip here that won't affect your planned stay,
they mention 15 months which means they got the initial 90 day Non-O visa and then got the year extension (3+12=15 months) so you're correct, they're comin' up to get a new yearly extension (y)
You can just exit the country, go apply for the DTV visa at a thai consulate in another country via the online eVisa website, then come back on the DTV
You do not need to cancel your extension of stay based on marriage (that you got from the immigration office here) before you go get the DTV