I'm not familiar with the video, but I'm guessing that it doesn't show you how to invert the wing to the bird's wrist. You need to buy a video that shows how to do this, but you basically can invert the wing by grabbing the armpit of the bird and pulling it to the elbow. It may take a couple of tiny cuts on a tendon or two to get it to this point. Once you are at the elbow, you can use your thumbnail and push the attached feather butts off the bone until you get down to the wrist. After you've done a few of them, the whole procedure to invert takes a minute. Once the wing is completely inverted, you can easily remove the meat and tendons. There is another way to do this where you leave the feathers attached to the bone, and it's all the same with the exception of once you're at the elbow, you remove the flesh with hemostats and leave the feather butts attached to the bone. I personally don't use this method, but would use it if I was going to enter a competition.
What will happen if you leave the meat in the wing? One, it will stink, two, it will attract bugs, but maybe with a bunch of Borax you could eliminate the second. I don't think you'll have too much of an issue with fats and oils leaching through, because there just isn't a whole lot in the wing, but it isn't too late to invert the wing and remove it properly.
I am surprised that the video doesn't show you how to do the inverting, because I'm guessing that it shows you how to use the artificial head, due to the supplies you ordered. With the video being modern enough to show you how to use the artificial head, I'd be surprised that it wouldn't show you how to invert the wing.
Buffleheads are a nice first bird. I also think they are a nice 2nd, 3rd, 4th, 5th, 6th, 7th, ..........The reason I say this is you have to get good at technique before you get good at anatomy, grooming, different poses, etc. etc. There really aren't too many birds that are as resilient as a Bufflehead, and they sort of come together on their own. By all means work with an easier specimen like Sea Ducks and Divers before you leap into Puddlers like Mallards, Woodies, and Teal. The other thing to mention on those little butterballs is they have a goodly amount of fat, and fleshing, washing and rinsing are probably the single most important steps in a quality mount. So if you're fleshing with scissors and a scraper, spend the money and buy an electric flesher.
Good luck, and if you have any questions along the way, please feel free to PM me or ask on this forum.