Tony,
Cavity backed clubs by their nature are designed primarily with reducing the slice or cut of the higher handicap player. Since you have played to a handicap of 3 in the past you are obviously no mug, the fact you state that you are the worst putter on the planet also suggests that you are a superb ball striker. I would imagine these cavity backed clubs are probably a bit of a blunt instrument for you. Cavity backs tend to be quite heavy and clumsy feeling because the extra forgiveness lets them sneak the shaft length out a half inch or so to make them a bit stronger distance wise.
As for swingweight, I think the general rule for any club is to play the lightest club you can control. Personally I like quite a heavy club, but I need the weight to be evenly distributed throughout the whole club, I hate some cavity backed irons where its like swinging a cannonball on the end of a rubber band. I have a set of smallish blade heads sitting waiting for shafts and I am thinking about putting a set of Tx-Tours on them, the whole club will be light but balanced (hopefully) so better clubhead speed can be gained.
I think I understand what you mean by rotation of the head. It feels as if the head is pulling the whole club along rather than being part of the swing. Its not nice, I tried a set of
Titleist 762's once and they were awful for that.
D.