RG, I think your B11/2 is the same dual sensor as my 103, it just has one common ground pin, thus the 3 poles vs my dual-ground 4-pole.
So, it would be the one controlling the KE unit with the cold-start injector, and the ignition. But, i agree, very likely not the cold start injector. I unhooked the entire fuel line to mine and plugged it with a cut-off bolt, and ran it that way for awhile - no difference.

The duty cycle readings will indicate correction for rich below 50%, and correction for lean above 50%, so a 90% read would indicate it's trying hard to richen up, maybe correcting for a vacuum leak as you suggested.
one thing, on a hot restart, the EHA remembers where it was running at idle before it was shut off. So, if it was really working to control a lean condition it would be enriching the mix a lot on open-loop start.
You may need to build the little harness to check the EHA values.

But, be careful on the connection to pins 2-3. on my Sears meter, the RED lead goes in pin 2, just backwards of what you would think. You can check it by looking at the % before you crank it, should read + 70% or some other positive % depending on your specific car.

The fuel accumulator has no prevision to control pressure at some value, it just stores what is there. I have always read in the manuals that the initial injector release pressure is around 3.2 BAR. I would think that if an injector is leaking a bit, and the pressure is slowly dropping, that the chugging would clear up pretty quickly after hot start, like mine does, within 5 seconds or so. But, I have replaced all injectors twice with no improvement in the chugging. so, I wrote that off. Maybe I'll pull them all out and reconnect them above the head and pressure the system and look.
but, once i actuall pulled all the plugs and blew out all the combustion chambers with compressed air, then quickly reinstalled the plugs and fired it up - chug, chug for about 5 seconds. Ho Hum.

I'll hang with you on this, you're a good guinea pig for more attempts in solving the mysterious problem.

DG