What did the surfaces of the pressure plate and flywheel look like when you had it apart. I've had my share of clutch chatter problems over the years. It is frustrating.
A broken drivers side motor mount will induce chatter, but I read you have a torque strap installed. Can you tighten that so that there is no play in it and it completely stops the driver's side of the engine from lifting up at all? That could eliminate any up and down movement of the engine as the clutch is let out.
I've also had bad pressure plates that show three or more hot spots where it is not evenly applying pressure to the entire clutch surface, and that induced chatter. You did change that out too I believe, and it altered the dynamic of your chatter. Is that correct?
Another time, I had a bad Marcel spring which is a wave style spring located between the two faces of the clutch disk was flat. It compresses when clamped between the pressure plate and the flywheel.
As mentioned above, a heat damaged flywheel can have both soft and hard areas that have different coefficients of friction can induce it too.
