Lots of possible explanations for the spare. Could be as simple as a flat tire someone was too lazy to put back on the car.

I don't think you have a toe problem if the car tracks straight and there isn't cupping or sawtooth wear on the tire. What's left is a camber problem because of worn bushings and/or worn springs. It comes down to whether you'd rather throw money at the suspension or more frequent tire changes. I'd be more inclined to address the suspension if left and right rear tires show different wear patterns. If both have comparable camber wear patterns, the $2000 you'd probably have to spend getting the rear suspension back to new condition will pay for a lot of rear tires. Even more if you swap tires left for right keeping the same direction of rotation. Tire shops will laugh at you for trying that.

The first thing I'd check is the subframe bushings. There's a bracket the shape of a salt shaker at the front ends of the subframe. The round part of the bracket is just below the forward subframe bushing. There should be about 1cm gap all the way around the bushing. If there's much less than 1cm or the gap is more of a wedge, it's time for new front subframe bushings. The rear mount is at the differential. The mount consists of a rubber block with two bolts into the differential. The block floats in a rubber lined mount. The rubber lining should have a slight bump at the very bottom of the lining. Maybe 3-4mm tall. If that bump is worn flat or if the block isn't centered in the mount, you need a new mount. IIRC there's a rubber biscuit or two between the top of the differential and the subframe. Those are a PITA to get to and I don't think they have much bearing on ride height. Replacing rear subframe bushings if about a half day job.

That should restore some ride height. Then there's trailing arm bushings and springs. If you go through the trouble of replacing the springs, might as well allocate time to inspect the trailing arm bushings and replace as necessary. Oh, there's also the rubber pads the springs sit on. MB provides different thicknesses of pads identified by the number of nibs.

Sixto
87 300D