Car has 28K miles, starting having vibration in the seat when pressing brakes. Pads are original to the car, in the front are at 60%, pads in the back at 90%. First, machined the rear rotors, did not solve the problem. Second, machined the front rotors, did not solve the problem. Finally, against advice of many experts, changed the pads, problem was gone. Question is: how would pads cause shaking?? One interesting observation is uneven wear at the front pads between the inside pad vs. the outside. Is there anything else that needs attention???

 

Pads can leave deposits on the surface of the rotors. Are you hard on the brakes? It is a heavy car as well.

 

The pad wear from front to rear seems somewhat normal. The fact that you had it shimmy and uneven pad wear suggests a caliper issue. The majority of times this is caused by the slide pins not being lubricate enough from the factory with high temp grease. Every time you do a pad change you should always inspect for pitting, clean off the pins and apply new high temp grease.
Also it is important to clean of the residual brake dust on the caliper.