1) Kind of. IIRC, if you carry the Sword of Chaos around, and take Sarevok into your party, it will upgrade for use by him (although you'll likely have better weapons by this point). It adds extra abilities when you beat each test in the pocket plane (which were originally in the game, they were removed to avoid confusing the player, NOT for balance reasons). And I believe Imoen gets something a little special as well
2) The Item Upgrade mod contains an upgrade for Celestial Fury (CF + a demon heart + a wand of lightning + 10k gp, IIRC, gives you CF+5- all the same as the original CF but with +5 to hit and 1d10+5 damage). Tactics has a Katana wielding Kensai that shows up and attacks you, he's got a good katana on him as well.
3)No clue.