I'm also going through the severe brake squeal symptoms on the rear brakes on my 98' C280 with single pin calipers.

I changed the pads (but not the rotors) a few months in the summer back with genuine MB pads (reused pins and spring clip as they were not available from FastLane parts, but they are fine with lots of tension on the spring clip), and had no problems and no squealing until around November. They are so loud now that they will hurt your ears, louder than the horn. I cleaned the calipers and added brake grease, no help. I then beveled pads on all sides and added the orange brake gummi to back of pads, still super loud. Then I put the car on jack stand and put the, held the gas (with traction control off) and brakes at varying levels, until it went away (disc were smoking hot melting the grease away I suspect). Took a while to get them to stop squealing on eth jack stands, and then I took it for a drive (to help cool the disc and pads) and no more squealing...Until the next day and now it’s loud again.

All that is left to do now is to order two new disc, and new pads to match.