Rear shocks have NO effect on alignment on the C-class.
Since I have worked in MB dealerships(retail outlet center) for almost 30 years, I will tell you that a spreader bar has little or no effect on alignments unless the front end is worn out! ALL 4 of the dealers that I worked at had spreader bars for final toe adjustment, BUT were never(hardly ever) used due to the fact we wouldn't align cars with worn out front ends.
Alignment machines are only as good as the tech who uses them & how often the equiptment is used & calibrated.
Some times at the racetrack we use STRINGS to check & set both front & rear toe-in. AND we can get them very close this way.
FROM the 1960's MB has recommended checking ride height before alignment, but now you must check ride height so that alignment specs can be applied.
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)