Actually, as seen in Star Trek: First Contact, Borg seem to able to go without breathing for extended periods of time, otherwise they would not have been able to construct a beacon on the Enterprise-E's exterior hull surface.
As for the matter of Borg vs. The Empire, that's a tougher question. The Empire's primary weapon is the laser, even oversized overpowered ones like the Turbolaser and Death Star Superlaser. We've seen on Star Trek that the Enterprise's shields pretty much shrug off lasers as a non-threat, so I'd imagine that the Borg shields tune them out automatically. I don't have the physics training to do the math for comparing the power required by a shield to block out the energy behind a planet-cracking Superlaser blast, though. If the new Star Trek Role-Playing Game coming out from Decipher is D20-compatible, we may just have to roll the dice in order to find out.
As far as Borg adapting to a lightsaber, Borg drones possess personal shield generators. I've never seen a lightsaber cut through a force-field, or Qui-Gon & Darth Maul would have kept going at it when they were on opposite sides of the shield in the Theed Palace reactor core. Also, if the Borg had assimilated some Cortosis ore into their structure, all bets are off. (Cortosis is the only known metal that reflects lightsaber energy. The Yinchorri made use of Cortosis shields in their campaign against the Republic in Star Wars: Jedi Council - Acts of War from Dark Horse Comics, and I suspect that it was a component of the Ancient Sith's alchemically-forged blades)
Regarding the Borg's location, they do not encompass the entire Galaxy. The Borg Collective is centralized in the Delta Quadrant (the Northeast quarter of the Galaxy. Earth and the Federation are located in the Alpha Quadrant to Galactic Southwest) where it is comprised of a large number of sectors and systems connected by a network of transwarp conduits (stable Hyperspace express-routes, so to speak). I'd say Borg territory is between ten to twenty thousand Light Years in diameter, a huge portion of the Delta Quadrant, but not all of it. Also, the Borg can cross the Galaxy in a matter of hours due to their transwarp conduit network, which is known to have terminals in the Alpha and Gamma Quadrants. Controlling territory in the Galaxy may be irrelevant if they can reach it in minutes anyway.
And with respect to Jedi Mind Tricks used against the Collective, again we'll just have to wait for gaming stats for the Borg Queen and roll the dice.
Only 400 Star Destroyers? I thought there were more than that, at least a thousand if you count the Victory-Class.
Oh, and about the look on Borg #3458485's face when he gets lightsabered? It would most likely be the same blank emotionless mask he always wore.