I agree with you regarding the wide stance. I wasnt/dont feel so comfortable with it but when i raised the subject with the instructor he mentioned that it was just a matter of getting used to it and that the extra width would help with the balance while sucrificing some maneuverability. Dont know about this topic so i guess i should leave it to the experts.
About instability...i cant really recall. Didnt focus on it and only thought about it when i started wiggling.
Just watching your video it seemed like you had a wider stance than I would have expected, but couldn't be sure. Now this discussion...
I think you should tinker with your stance. Narrow it by an inch or two and then fine tune it (narrower or wider from there based on feel). Stance is body structure AND board type related, in my opinion. At 6 feet, I rocked a 19.5 inch stance on my old camber board which is ridiculously narrow for someone with a 35 inch inseam, but that's what worked best for me on that board - better stability and turnability, less edge catch. On my Arbor Westmark rocker board, my stance is 21-22 inches to give me the best stability and turning response.
Due to the rising cost of ammo, do not expect a warning shot! Sorry for the inconvenience.