I have found a solution to my problem and to any other carbon bike owners who would like to use this Thule Tour Rack. I basically turned the disadvantage (downward sliding force) into an advantage.
But before that I did try the rack again but this time adding in a rubber padding between the frame and straps. I cut to size an old punctured rubber tire tube. Made it a little longer so it would wrap around the frame 3 times for added cushion. Use a piece of heavy duty double sided tape to keep the rubber wrap in place. Installed the rack and it stayed on perfectly. I placed a piece of tape beside the strappings so if there were any movement a gap would form. I have no idea why Thule do not or stopped including some sort of rubber padding. Think about it-if you want to open a tight jar lid, which material would provide more surface traction? 1) Nylon strap with 20% rubber contact or 2) 100% Rubber strap. There are these rubber wrench you can buy at hardware stores. These rubber wrenches have thick sticky like rubber straps not nylon straps.
This is a clean look and would be fine for anyone with a non carbon bike. But carbon bike owners have to worry about unintentional forces being applied to the carbon frame. Especially concerning since the carbon seat stays frame is rather slim/small relative to the rest of the frame.
The problem with the above solution for carbon bike owners is that now all the force applied by the weight of the rack, pannier and contents is being transferred 100% to the thin carbon seat stays. To reduce the load on the seat stays I want some of the force to be applied perpendicular to the seat stay but also parallel as well. But this parallel force causes the rack to slide down. So I re installed the rack without the rubber padding as I did the first time because I actually do want the rack to potentially slide down. Additionally, this time I did not tighten the ratchet as tightly the first time because I want the rack to have some sliding force. This is also better because there is now less compression force applied by the straps around the carbon seat stays.
To prevent the rack from sliding down I attached a 1" arno nylon strap around the Thule aluminum frame attached to the bottom most nylon strap ratchet. I did this to both sides. To securely attach the arno straps to the Thule racket, I unscrewed the bolts at the bottom of the rack's aluminium rails. I used a eyelit plier and installed an metal eyelit ring on the arno straps. Insert the screw threw the eyelit ring and tighten. I had to buy longer screws to accommodate the width increase due to eyelet rings.
The 1" arno straps runs up parallel to the seat stays and attaches just below the bicycle frame's seat clamps. Now this thick strong frame just below the seat clamp holds the rack in place and prevents it from sliding down the seat stays. The bicycle frame below the seat clamp is very strong as its a T joint and designed to bear the weight of a 200+lbs rider.
Addin-g the 2 arno straps adds another load bearing point into the equation. The rack is now a 3 point load bearing system. Instead of relying solely on the poor thin seat stays it now has the much thicker and stronger seat post frame to help bear the load. Using the arno straps also lets you apply less compression force by the Thule ratchet around the seat stay.
I tested this around the park and it was rock solid. I can feel the tension around the arno straps doing its job of distributing load. The Thule ratchet is tight around the seat stay just enough to provide stability but does not have to be so tight now where it has to not only provide stability but also bear 100% of the load.
I tested this with both panniers installed. The weight of the panniers with its content was about 10lbs each. The Thule commuter panniers weighs 3lbs by itself. I think any carbon bike with the rack installed in this manner can safely handle 20-30lbs of total rear rack weight. The Thule Tour Rack and its Communter Panniers are specified for 40lbs load on each side but I would never test that on a carbon bike that has not been designed to carry a rear rack.
Installed in this manner, I believe the Thule Tour Rack is the perfect solution for all carbon bike owners who only want to carry light loads. Not because the rack and pannier cannot handle more loads but because the carbon bike is just not designed to handle a rear rack and its load. Any carbon bike that hasn't been designed to carry any rear rack should still be able to handle 20lbs safely. I used to carry 30lbs in my backpack but now I carry a 10lb backpack and 10lbs in both panniers each. This will relieve the soreness in my shoulders, arms, neck, lower back from carrying a 30lb backpack.
Its great to have a rack option installed on my bike now. The Thule Tour Rack looks great and their Commuter Panniers looks very stylish and works perfectly. Looks very professional, smart and mature.