Grondie could clarify the Rivendell-Grey Havens road better since he's a map freak. :)
I don't think the hobbits & Elves would have interacted that much around the war of the Ring period. The Shire Hobbits were a pretty insular bunch of people & frowned upon any outside contact with Elves, Men & Dwarves. They weren't always so isolated - I think they interacted pretty closely with other races right up to the 1650s, when the Great Plague wiped out almost all the hobbits outside of Bree/Shire area. The ones left didn't really see much reason to maintain contact with the outside world. The wars with Angband (some Shire areas were destroyed) also reinforced their isolation from the rest of ME. The modern Bree Hobbits were more cosmopolitan though - they interacted freely with Men (wasn't Nob a hobbit)?
As for the Elves - well, I think they knew more about the hobbits than vice-versa. Didn't Gildor say to Frodo - "We know (or have seen you) before" or something like that, and knew about Frodo's intended journey (from Gandalf, I think). Of course, don't have the book here, can't quote exact passages.
The One Ring - I think the Ring was only fully activated if it was worn by someone. Otherwise the Nazhul could vaguely sense it, but not truly confirm its exact location. The Nazghul Lord only saw Frodo when he had the ring on (Weathertop scene) but not, as you said, on the Old Road before they reached Farmer Maggot's mushroom farms.
The Black Riders - dunno, they were too weak at that time, maybe? Sauron had not regained his full strengh yet (no ring), so his ability to strengthen or influence his minions decreased relative to their distance from him.