It appears you already know compressor pulley clutches can be noisy but that alone shouldnt affect fuel consumption unless the compressor itself is faulty .Which would vary depending on whether air con is on or off.
I'm not an expert and there may be other reasons ,but I still suspect Brakes. Its not enough just to change pads and disc rotors, unless the brake calipers have been thoroughly inspected to ensure the moving parts are sliding smoothly. Sliding pins and hydraulic pistons can jam or partially jam within their bores due to road dirt,brake dust and corrosion. . They may apply the brakes well enough under high hydraulic pressure, but not retract as smoothly. They can sometimes be cleaned up, but might need replacement. . If the rotors are showing signs of wear or other problems they may need to be changed, and might improve things .But bear in mind excessive wear or damage to discs might be the result of other problems with the brakes, not the root cause of them. Especially if one side of the car is worse than the other.
You might think a professional mechanic would check all this stuff, but I once watched a brake specialist replace discs and pads. He used a professional high pressure hydraulic brake piston retractor to push the pistons deeper into the calipers to accommodate thicker pads but made no real check on the condition of the pistons or bore. Maybe he would have noticed any problem.
I'd also check the handbrake mechanisms along their entire length. Partially seized cables, stiff or seized linkage swivels ,missing release springs (if any are used) ) etc can cause the handbrake to remain partially applied. Again these may improve with some cleaning up and maybe some limited and carefully targeted lubrication.
One diagnostic check you could try is checking the temperature of your brake discs after a period of driving without having used brakes very much. If any particular disc (s) is hotter than the others ,eg rears hotter than fronts, or one side of the car hotter than the other it may indicate a brake binding problem. Be careful . The discs will be hot and can easily burn your fingers but a fraction of a second touch with a wetted finger should suffice to test if one is significantly hotter than others , with little risk of injury. At your own risk
If you still suspect the problem might be related to engine ancillaries you could check the the alternator and water pump are turning smoothly. .
