Hey smp

Ok, I finally remembered my password, been since 2008. :-)

Watched your vid, that is not a stumble, damn she falls on her face. Just some ideas since we are systematically troubleshooting. :-) I read through what you have done so far, actually you're ahead the game, good going.

1. EGR - yup if it won't hold a Vacuum it's done. Don't know if cleaning would do any good. You can also pull the vac line, depress and then hold you finger over the vac line fitting and the diaphragm should hold open. But, the EGR only is working when fully warm. Disclaimer, I own a 2.3 and thing are the same for a 2.6 but the way they get there can be very different. I see yours attaches to a box on the firewall where as mine is fed over the valve cover and up front to a Vac/Temp valve. When cold, the Vac/temp valve is closed anyway. Test is to rev UP and quickly pull the vac line and you should hear the EGR slam shut, takes a few times to be sure. But for now, you have blocked it off.

2. 50% Duty Cycle - is that fluctuating around 50% or locked. She must fluctuate once full warm. Locked at 50 is an O2 fault. I know you replaced it, might have a bad connection??

3. Vac - 13-15 is low with a real vac gauge on a running engine. I can't relate to a static vac using a pump. Can you get a reading with the mity vac with the engine running, or it's only a pump. I don't see how a pump could even work, you have a great big leak at the Air Flow sensor so I must be misunderstanding how you are using it.
14-17 and a steady Needle implies a Timing Issue. Good is 17-22 Hg on a healthy engine and your compression numbers look real good.
Cool link on how to use a Vac gauge, old school stuff before OB2 code readers.
Intake Manifold Vacuum Tests


4. I don't think just one injector could cause this, meaning your #6. This is a major air/fuel issue.
Here is an interesting link almost exactly how you are going about it. He found the air flow boot under the Air Flow sensor bad. You can see how he was chasing just as you are. Good possibility. I dont' know what MB he has. The manual pics are really bad. You can see the part at ********az 2.6 fuel/air Air boot.
Hesitation, KE-Jetronic problem? - Page 2 - MBClub UK - Bringing together Mercedes Enthusiasts

I think I would start with a smoke test. $1 cigar and 2-3 feet of vac hose that will fit on the fitting you found on the intake manifold. Block off the intake, mmmm tape or saran wrap/rubber band or a surgical rubber glove. block off the hose/ valve cover to air cleaner.

You can build a smoke tester from a garden hose sprayer but a cigar works very well. Take 4,5 or 6 blows, don't have to blow hard just give it some time.

Next plan we already discussed the importance of a smooth air flow sensor, must be clean. I am thinking IF not an air leak then the air flow sensor and it's relationship with the potentiometer and even the EHA.

Seems by the vid your ICV is not functioning 100% either but it might just be this Air Boot and the ICV can't keep up.