The specialist recommended by the Xfinity guys stopped by this morning, as promised. He was the quintessential grizzled old guy, who has seen everything.
He walked thru the house and listened as I explained the two routes into the basement that I had discussed with the young guys (who threw up their hands and said they didn't know how to make either work, guaranteed).
45 Minutes later, he was done, having taken a 3rd path, entering the basement a couple of feet from the aborted one that they explored the other day. Labor + materials + 30 years experience = $55. Such a deal.
So, now we're back on track for the full install, which is scheduled for next Tuesday. For the record, the guy's name is Ken Taliaferro, at 503-453-5025. Call him for any kind of custom wiring problems - glad I did.