Don't be too discouraged with the results, I am guessing it is an improvement, even though it didn't seem like it. The only way that you are going to screw up your aerodynamics by adding a sweet kammback like that, is if you had such good tail end aerodynamics, that adding a 10-15 degree extension somehow disrupted the separation point... you could test where air is separating from your rear window with another tuft test on your rear window. If it separates above the lowest point of your Kammback, the Kammback is probably helping, if it separates below, it's probably hurting. By looking at your car's tail end, I don't think you have attached flow on your rear glass... it looks too similar to the rear end on my car! The angle looks good, it is a similar angle to what I used on my
SedanKamm, and mine was a big gain! Mine was 4' long, and good for a 6% increase in Fuel Economy.
From your video, you have attached flow, which is what you want. This means you do not have to go more shallow on your angle, and could in fact probably go steeper! (I would keep it where it is) In all likelihood, the wiggle you are seeing on the sides is from having mirrors sticking off the side of your car.