If you activate the valve (which is possible either by using the "activations" screen with their computer or by applying vacuum directly to the valve) the engine should almost die.
OK, the test he mentioned, where he mentioned the "SDS", that's "sort of" positive news. SDS is Star Diagnosis System, it's a Mercedes test computer, so it's good to see they apparently have that shop tool, that's an encouraging sign. As I mentioned, they can use this to go "into" the engine control unit and as a test, activate the EGR valve. At that point, what's being done is this EGR switchover valve directs vacuum to the EGR valv (no need to use a mityvac then, at least not at this point). If they simply observe the valve while they do this, they can see if the valve is opening or not. If it is, you've just eliminated a whole lot of testing. The main thing is: "Did the engine start to run terribly or not?" If NOT, then more than likely it's the tube. I can't even think right now WHAT ELSE it could be. If it DOES run like crap, then something else is going on, possibly the engine control unit has some error in it preventing the EGR system from activating under "normal" driving circumstances, but will work OK with the shop computer (SDS) telling it to activate. You're an aerospace engineer, I'm sure this isn't over your head. If however they did this test with the computer, and the EGR valve DIDN'T open, then you have more work to do, either no vacuum to the switchover valve (which in many case means you have a vacuum leak, which means even MORE trouble codes) or a bad switchover valve, or the computer output to the valve isn't working, or hey, guess what? *Bad EGR valve*! So that's kind of the whole run-down of the system, you should have a fairly good working knowledge of the system, maybe as good as the guy working on it, (except all the principals of when the control unit activates it and all that stuff, I don't need to get into that, do I?
YES it's sort of a tedious job, it's quite a it more work than replacing just the valve, for sure. That's why you want to really think about your plans for the car (couple more years? a decade? want to will it to your relatives?). Again the new pipe should "fix" the problem, the part that goes inside the intake is longer. Cleaning it out? That's only going to last awhile, my opinion is if the car is an 89, it took 14 years to plug up (assuming it is), it'll probably plug up again in alot less than 14 years, they won't be able to make it as clean as it was in 89 and it'll reaccumulate faster. This tube does run from the exhaust manifold on the right side (passenger side), around the rear of the block, and to about the middle of the intake manifold. Once you've replaced one 3 or 4 times you get the hang of it and can do it in maybe around an hour, hour and a half, those first few times were taking me I believe over 2 hours though.