Some modifications/ideas you might consider for your next effort...
1) I'd consider making the fins a bit larger... maybe increasing their area as much as 25%, to make them more effective.
2) Either cant the fins slightly (only a degree or two) or make one side of the fin curved (like an airfoil, it should spin TOWARDS the curved surface) to put rotation on the rocket in flight. This will help in several ways.
a) The rocket will be more stable and less likely to "weathercock" like the last flight in the video.
b) The rotation will help the rotor blades deploy when they're released. (centripetal force)
c) The rocket won't depend on the blades to begin rotation, it will already be established, This will PROBABLY help you to get the thing spinning and slowing the drop sooner.
3) Make the motor mount slide back when the ejection charges fires. This will move the CG of the rocket rearward, and you can use this movement to release the blades if you make latches on the mount that hold the tips of the blades.
4) Use some kind of spring(s) or rubber band(s) inside the rocket body at the top to pull hooks on the tops of the blades to the center when they're released. This will be a kind of "booster" to get the rotors deployed without having to rely on it falling tail-first, and air doing the job. If necessary, increase the diameter of the rocket body around the point where the rotors are hinged... there are any number of real and model rockets whose diameter just below the nose cone is significantly larger than the rest of the body.
You might try some drop tests with the rockets you have, too... stick an expended engine in them. rig the rotors with thread or tape to hold them open, and try dropping it tail-first out of a second or third story window. Give it a little spin to get it started, too. That will help you verify that the rotors actually will do what you want them to.
I REALLY want to see this thing working right!