I would really contemplate trying someone else with the problem, unless you can threaten a little with going elsewhere and have the shop owner concede that something is "a-miss" and figure out what happened. By the sound of it, my guess would be a goofed up cap and rotor change. Did/do they use MB or at least aftermarket Bosch products? Possibly 2 of the plug wires were reversed when putting the wires onto the cap, believe it or not they will run like this. OR the center contact in the ignition cap could have been damaged when installing.
I think the shuddering is just being caused by the engine missing when pulling to a stop due to the miss in the engine, more noticeable at lower RPM's. The motor mounts may be wearing out, but no reason to notice that it's worse just because the engine was tuned up.
Another thought is that you can try gently pulling on each plug wire to make sure they are all snapped into place and not loose, this can cause a miss as well. They can't hardly be put onto the wrong plug (on the cap they can, as I mentioned already) but maybe not put on tightly. The torque on the plugs is 20nm, is you want you could doublecheck to make sure the plugs are all fairly tight, a loose plug could also cause a miss. Spark plug gap on all but the latest engines is .032". I recommend the H9DCO plugs, only difference is a heavy duty center electrode, the H9DC is fine though. I forget what year your car is, but in April 1988 the H9DCO became the recommended plug anyways.
Checking the on-off ratio is a good idea, but I would try approaching this from the standpoint that you now have these problems that are new, were they caused by the service work?