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Curious as to what others might think is the biggest contributor to this mess:
Kind of thinking that someone didn't install a gasket between the mixing pipe and the intake: I know that these gaskets cannot be found now; perhaps that's been the case for several years (this engine was, supposedly, installed as a replacement about 4 years ago). I do not recall seeing a gasket when I pulled things apart (it's been a couple of months ago) and I am unable to locate one now (so I'm thinking that there just wasn't one to begin with).
Here's a close-up of the other engine's mixing pipe to intake where the gasket is readily visible:
Curious whether the resonator seal could also play a part in a mess like this. I have run across a part number of 013 997 84 45. I have a new o-ring from the charge pipe to the mixing chamber, so am good to go there.