So Borg cube wreckage can "safely" land on Earth without killing its inhabitants, but Columbia couldn't? I guess those nano-powered cyborg implants are resilient, indeed.

Why didn't they call themselves "The Borg" in this episode? So the show's producers could avoid looking desperate for a tie-in to a more audience-alluring enemy from an entirely different era?

By the way, as I recall from 15 or so years ago, the Borg discovered us when Q mischievously sent us way out into perhaps the Delta Quadrant. Perhaps they never received the signal that Captain Archer discussed last night. But then again, as you keenly & insightfully mentioned, perhaps the timeline's simply altered now and future Borg DO indeed learn of Earth as a result of cousin-Borg that ventured back in time. What an intellectual roller coaster those Borg can put one on at times ;-)