The belt separation that was noted by the mechanic is the circumferential belt that forms part of the tire internal structure. When this structural element loses its bond to the adjacent structural elements the tire expands due to centrifugal loading at high speeds in an uneven fashion, making the tire go out of round. This is a characteristic of a poorly designed and manufactured tire in my opinion, caused by the engineering approach favoring lighter weight over robustness, or using new, unproven manufacturing methods. When this happens it is approximately the equivalent of a metal fatigue failure, except it is in the complex structural composite of a tire. Anyway, I do not use Pirelli tires anymore as I found them unusually prone to this kind of failure. My 1986 190E 2.3-16 came with Pirelli P600's (Z rated) and they suffered a belt separation within a year and 12,000 miles. I spent quite a bit of time at 80 mph and higher, which made the condition intolerable. The tires would balance ok on a machine in a shop where they may spin at 35 to 45 mph, but on the car at 70 they would start to shake. After trying a number of brands and performance levels of tires, I have settled on Michelin Pilot XGT-4 types in an appropriate speed rating for each car (H, V or Z) and have never had this kind of problem again.
Back to your problem. You do have the same kinds, or equivalent parts in your steering as the 280. And I have changed a number of these bits and parts before to correct play in the steering. The difference between play and floating however is probably the key to figuring out what your problem is.
With the car running and the emergency brake on, or the transmission in Park, set the wheels in the straight ahead position. Then turn the wheel one direction until you feel resistance, and back the other way until you feel resistance. In a car with a tight steering box and linkage, this should be approximately one quarter to half an inch of circumferential travel. Mercedes allows up to an inch before they consider the play excessive.
With the alignment within specs, and properly functioning shocks, play in the steering is readily distinguished from the floating issue. The car will still track in a straight line, you just have a no "on-center" feel with excessive play. When you go through the inch of play, it almost feels like the steering wheel is not connected to anything. Once you take up the play though, the steering should feel quite normal, until you get into a cross wind or a change of road camber that requires that you quickly turn in the other direction and go through the dead zone again. This condition is apparent at all speeds. Is this one of your problems? To see if it is in the steering box or the rest of the steering links, you need to check the steering wheel motion into the box vs. the steering arm motion coming out of the box. There should be only a few degrees of free rotation of the steering wheel input shaft before the output starts to move.
I am not familiar with the action you describe as you get to the end of the extreme left or right turning capability of the car. I have never experienced such a condition, or I may be misunderstanding what you meant. I believe you are saying the rate of steering response to turning the wheel lessens as you approach the end of travel in each direction. I do not think this is built into the geometry of the steering system, so if it is happening, it might be related to the power steering pump performance. Dirty fluid can clog the filter, starve the pump, and, until you turn the wheel fast you will not notice the effects. The filter and fluid should be changed/flushed before you attempt to adjust the steering box.
The causes of floating as I have experienced it are related to a combination of misalignment, tire unbalance, weak shock absorbers, and worn steering hardware that maintains the correct geometrical relationship of the front wheels as they are steered through the turn. A good inspection by an alignment specialist should uncover any of these issues. If you have worn steering links and bushings, or rear links and bushings, you will not be able to align the car and get repeatable measurements. A good technician will examine the front end before he starts the job, and will take readings after turning the wheel each way before tackling the adjustment features with a wrench.
Sorry to be so long winded, but my 190E 2.3-16 has over 200,000 miles, and with regular power steering fluid/filter changes, and two sets of front shocks (the first ones were under warranty from the dealer in Germany) the steering is wonderfully tight and the car fun to drive. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)