Scuffing pads is normal. That the scuffing is similar on both sides speaks highly to the evenness of your braking system.

Vibration due to brakes should only be noticeable in motion during braking, not when driving or coasting. Brake vibration is usually magnified when the brakes are hot, such as coming to a stop light on the highway. When cold, as you come to a slow stop, feather the brakes and you will quickly know up through the steering column whether "warped" (uneven) rotors are the issue with pulsation keeping in time with wheel revolution. Worn/binding calipers and loose/binding pads themselves should not cause vibration, but serve to magnify warped rotor symptoms under braking. Brake squeal is a whole other topic.

Your noticing brake pad scuffing for part of the revolution when the wheels are being turned is quite normal. You can only expect scuffing not to be present (or a minimal scuffing all the way around) when the rotors are pristeen and within the .0004" thickness variation spec and have been indexed/shimmed to the hub within the .0014" runout spec. As brakes become used, uneven pad deposits on the rotors will lead to hot spots, uneven expansion, uneven wear and consequent "warping". I'm just now learning after all these years the importance of occasionally re-bedding the pads before problems start to develop, especially with city driven cars.

As for seizing calipers there are two issues: the pistons not retracting after brake release or the caliper guide pins binding. After a good long drive, check the hubs with your hand to see if they're notably hot, compared side to side and to the rears, an indication of dragging brakes needing attention.

To check for adequate piston release, raise the front wheels, apply the brakes firmly under brake boost then release. With effort, you should be able turn the wheel gripping the hub area or with one finger on the outside of the tire. With the wheel off, you should be able to move the rotor around by the studs using two hands. As long as you can do that, it's safe to assume the pistons are able to release and will further release under travel. When the pistons and seals are pristeen the pistons should pull back enough on their own so the wheel and rotor can be easily turned. But as things go along the seals will start to wear and not retract the pistons as far, also dirt collects on the pistons and restricts piston movement under the seals. Eventually the caliper will need a rebuild with new seals, or as most do a reconditioned replacement.

Although the guide pins can bind and occasionally lockup (to the point of not being able to drive), more commonly the caliper simply starts to get a bit sloppy and you will note uneven pad wear and uneven rotor wear. Removing and relubing the guide pins (and occasionally cleaning out the guide recesses) with high temp silcone brake grease at each rotor change is advised and optionally at every pad change. Worn guide pins can be replaced, especially if rusty. The guide bushings in the caliper frames wear just as much and are not replaceable except at a machine shop using a drill and press, also needing to source the bushings as they are not available in the Volvo parts chain.