Each car is different when it comes to how much road force variation tolerance there is… The 26 lbs spec the shop has provided you is just an all around maximum average… Some vehicles can handle significantly more, others can only handle force variations in the upper single digits. In my opinion, 26 lbs is too much for some pickup trucks to handle, let alone a car.
Another issue to keep in mind is that you have three tires in the 20 plus force variation range… Each tire vibration produces a sine wave (Graphical view of the vibration over time) There are times where the peaks of each sign wave match up with each other… its like putting batteries together in series… the amplitudes stack up… When this happens the vibrations can become severe and when out of phase, feel not as strong.
Excessive Road force variation will result in a rhythmic vibration that is directly proportional to the rotation speed of the tires. If we think about it, if one tire has a one hard spot or high spot, the tire will create a vibration one time for each rotation of the wheel… if there are 2 hards spots that for each tire rotation, there will be two vibration pulses. When you say your seat bounces… I am assuming it is a continuous shaking/vibration?
Typically runout issues will generate slower speed vibration like you have indicated… This would include bent rims, bent axles, improper wheel/hub centering. etc. Again there are exceptions, each car can respond a bit differently. Yes excessive road force variation due to just the tires can also create such vibration.
The GSP9700 balancer can be an art form to use properly… Yes it shows each wheel/tire assembly out of balance level as well as force variation, it can even show you whether the rim itself is any good (Bent, out of round) Setup is critical when using these machines, there are proper wheel to machine adaptors that are required for proper wheel centering. Tires may need to be match mounted to the rims to acquire the best road force variation numbers. All of these requires a trained operator… Today tire balancing has become an art and science, not a job just anyone can do anymore. The GSP9700 also will provide the road force variation print outs for each wheel tire assembly tested. (Did you get them) Were the tires force matched?
I have a few more questions for you…
One: how long has the vibration been there, i know you have your not sure exactly how long it has been. Was there any change in the vibration characteristics with the new tires (Better/worse)?
Two: Is the vibration worse first thing in the morning when the vehicle/tires are cold? Does the vibration diminish as you drive?
Three: Can you feel the vibration in the steering wheel or is it just in the vehicle body/seats? Has performing a wheel/tire rotation changed the vibration characteristics?