1 - you're correct you'll apply for a year long extension based on marriage to a thai and you will get a 21 day under consideration stamp (C/W does 3 week stamp)
That under consideration stamp has NOTHING to do with the current stamp you're on when you cancelled your employment extension two different stamps for completely different reasons.
2 - I would say you need to dress politely for the photos, Re-take them, you, your wife standing in front of the address numbers, you and her in the house at the kitchen table, in the living room, etc.
3 - doesn't matter when you go, you should make an appt, at this website
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4 - correct if you're out of the country when this extension expires it runs out and you'd start over from zero either by getting the Non-O visa before you come back or comin' in on a free entry stamp and doing the whole process inside the country