OK - it's been solved!

First, I did start with the barrel adjuster, but when I couldn't adjust it fully and went into the LBS the last time (before today), I was scolded for letting it get too far using the barrel adjusters.

Anyway, here's the situation: Ironhorse has been sending too-short cable housings. Typically, apparently, bike manufacturers send all the same length cable-housings, no matter the size of the bike. It seems that Ironhorse is now customizing their housings, but it turns out that they're too short.

So, when I make sharp turns (never) or turn the handlebars to fit the bike in my car (always), the too-short housing was thrashing the cable. The guy at the LBC thought he was going crazy with being unable to adjust the cable properly until he happened to notice the cable housings and exclaimed "they put the bike together wrong!" (Again, though, it turns out it wasn't put together wrong, but outfitted poorly from the manufacturer.)

They fixed it today and replaced the cables and housings free of charge.

Yay!

(It was cute, though. The mechanic must have sensed how neurotic this all was making me, because - after he replaced the cables - he made a point of explaining that the cable would now need to be stretched for a while, so I should expect to have to bring it back in to be adjusted. I guess he was afraid that I would come in next time in tears. )

Thanks again for the help!