Hey @vac, great that it’s given you some insight. Like everything, I think that these exercises can only give you clues about what’s going on with your playing. I still use this stuff to keep track of what’s happening. (Recently-ish I’ve switched to gypsy jazz style technique, and am still running into issues with tension etc – neverending, for me, at least.)
I hear you on the forearm-in-the-air problem. It’s possible that you’re using too much effort to hold the entire arm up. Maybe try some of the arm drops, just to get a feel for how little effort you need to support your arm. Sometimes when I lift my arm, I find that my wrist gets involved too for some reason, even though it has nothing to do with supporting the arm. It’s like the hand tries to lead the lift, when in fact it’s all done by the elbow. And I don’t think that your shoulder needs to be involved much … sometimes I find my entire shoulder lifting when my arm moves. So, you could get your arm into position, try some single string stuff, and then simply try to drop the arm so it free-falls down to hang by your side. It’s not a realistic approach for real playing, but it may give you an idea as to how much extraneous stuff is getting involved in your arm positioning.
Switching between slow and fast playing can help as well, because when you play at speed it’s not so much a feeling of “tension” or “relaxation” but one of “properly-tuned engagement”, if that makes sense. Especially when you switch strings, only a few things need to happen for your arm to get repositioned correctly, and if you use too much effort it will quickly build up and lock things out.
So, I’d say just keep working at it, bit by bit. This is new movement acquisition, rather than something that you grind out over and over. Try it out for 10 mins, take a break or switch to something else, then 10 mins again, etc etc, taking it easy and just keep exploring.
ps - everything is just a theory and an idea. Keep hacking at it if it works for you. Cheers! jz