Peraonally I don't think that "instant feedback" is nessisarily the way to go when it comes to improving form. Yes, having a buddy film you on occasion helps bring the focus to the technical elements that you might be struggling with and what errors you are making but when it comes to working on them, you actually have to work on them.
If you think about freediving like any other sport you can taking boxing as an example. You don't show up and start fighting while your coach tells you to tuck your chin and keep your hands up. You do drills focusing just on footwork, punch mechanics, head movement, and after working on those things you bring them together to have a sparring session/ fight.
Using that logic if we look at duck dives you should dedicate time to working on duck dives, not doing CWT. I worked on duck dives by practicing them Before or after a dive session. The way to get the mechanics right is actually to make the duck dive harder that it ever will be.. to do this you can remove all of your weights, duck dive without using your arms, try slow motion duck dives... after a few session of practice like that I could do a duck dive without weights, without my arms, and after packing 1-2l extra air. If you can do that the mechanics are right.
The same thing applies to finning, once you know what your supposed to be doing with your fining take time to work on it. Use training fins and do shallow/short dives. Once the mechanics are right you can work on optimal power/frequency/amplitude. This can be achieved by doing easy (20-30m) dives with out weight, or with double weight. This forces efficient fining and you'll feel right away if the technique isn't optimal.
With free fall you can do 2 things.. while falling adjust your body to slow yourself down and to speed your self up. The fastest speed you can get is the most streamlined. The other thing you can do is find the most comfortable position. By mixing those two positions you can find something that is comfortable and relatively fast as well. That position is the most optimal and can be achieved entirely by feel.
So yes. Having someone watching and filming you (given that they have a deep understanding of the positions and mechanics) is nice and can be very helpful.. that being said the long term way to keep improving is by dedicating time to isolating and practicing each element and by adding extra resistance (too much weight, no-arms duck dives) to make each thing harder, forcing yourself to find optimal technique.
