I was always taught to support the weight of my arms with my fingers. I think that it's possible that, at times, particularly when I'm learning something, I am unconsciously pushing down. Perhaps this will sound odd, but I tend to think of tension as locking my arms so that their weight is NOT going into the keys, so in an attempt to force relaxation (obviously an oxymoron) I push down- use excessive force, as you say.
In sports we see the same thing. Golfers are told, "Keep your head down," when the point is actually not to lift it. Then beginners force their heads down with resulting muscular tension that they fight against with the opposite muscles.
I think the spreading of the fingers was making my forearms feel tense, so perhaps I was trying to create the illusion of relaxation by making sure plenty of weight was sinking into the keys.
I think before I go back to Chopin I had better let the arm get close to full recovery- perhaps I'll go back to Bach fugues for a while and maybe try some left hand etudes to work on technique a bit more.
As I write this I realize that for me the best way to avoid tension has been simply not to think of playing as a physical challenge, but rather to listen ever more closely to the sound being produced and to try to bring it closer to what I want. I think that in that sixths passage I don't HAVE a clear idea of what I want. In the rest of the etude I know what I'm trying to do (for better or worse!) but in that passage I'm just waiting for something to happen, so it becomes physical. Iws that too goofy an idea?