As stated in my first post: the chain is NOS: New Old Stock; still sealed in package, never used, brand-spanking-new. As are the cassette, RD (& its pulleys), BB, & RH. The FD is VGC, little wear; STIs are newly rebuilt (& worked like a charm on my other bike).
The only weak point was that outer chainring, which- from its markings- the Germans quickly identified as being from an earlier 1050 (UG) crankset. Despite having pins, the UG ring is still mostly flat, whereas the SG outer chainrings are also slightly dished to accommodate the shift ramps on the inner side. This accounted for the millimeter or two difference in the chainline that caused the chain to rub against the back of the top ring.

The thing that threw me about the 1050 ring was its similarity to the inner SG A-42, which has a similar finish and is also mostly flat (at first they look like they should be a matched set); then it dawned on me: it's an inner ring for a double; it doesn't have to have ramps on the back.
Another puzzle was the bolt-tabs on the 6400 crank: the thicker 1050 ring fit snugly, whereas the SG inner ring looked too thin (ergo, why I thought it might be the problem instead of the outer ring); then I read that for the first year or two, the 6400 cranks were issued with thicker 1/8" UG rings, and only later with the 3 3/2" SG rings; the crank will actually accommodate both ring types.

As for cross-chaining in general, yeah- it's not the best thing. I've been telling my best friend that for 14 years and he's still too much of a retrogrouch to bother upgrading the 1055 chainrings on his RB-1 to match his HG cassette & chain. Instead, he just lubes the crap out of it & hopes for the best.

So, to recap, I knew what was going on; I just didn't know why. Then I did; now it's fixed: Das Ende.