Bear in mind that BG2 starts only 1 year after the start of BG1.
Unique BG1 items that definitely could not have gotten to their location in BG2:
Gauntlets of Ogre Power, Bala's Axe (added by UB): The Planar Sphere is completely sealed against everything, and hasn't even been on this plane for the past 500 years. Nuff said.
Unique BG1 items that probably could not have gotten to their location in BG2:
Ashideena: Unless the hammer was taken from your person, then given to one of Irenicus's Duergar or a Shadow Thief as payment for helping to capture you, and the bearer then became an early convert to Gaal's little cult and went down to seek the other half of the Rift Device, answered the riddles to make the bridge whole, got across, suddenly died, and the bridge broke in half again, I don't really see how Ashideena could have found its way into that container.
Suryris's Blade: Odren says that not even the Knights of the Vigil are able to refresh the seals on the various levels, and I believe he also implies that nobody has even ventured into the Keep for years. I believe the halberd is also sold by Ribald: a far more appropriate placement, methinks.
Cloak of Protection+2: So--it was given to a Shadow Thief, who was then assigned some mission that involved a trip to the Pirate Isles, his ship was sunk in one way or another, and the Cloak was found/taken by Ixilthetocal, who, instead of doffing his Cloak+1 and replacing it with the +2, instead opened up a chest which contains a soul-consuming Spectre, stuffed the Cloak inside, closed and locked the chest again, then played "Eye spy" with the Imps. Me, I say it's simply not the same Cloak.
Favorites:
Varscona: I've already said that my favorite Varscona placement is the new store added in aVENGER's Rogue Rebalancing mod, and I think she also sells Grave Binder.
Spider's Bane: Reintroduced in UB (Pai'Na's quest), Tactics (Improved North Forest), and Rogue Rebalancing (Rogue Shop)--with the UB version being my favorite, as well as being actual UB.
Other stuff on your list:
Staff Spear+2: Dropped by Xzar.
Staff Mace+2: Dropped by Haz in Bodhi's Lair and/or Aran's Guildhall, and also by the Mage next to him in Aran's Guildhall.
Kiel's Morningstar: Placed by Tactics (Improved North Forest) in the grave in the North Forest, along with Kiel's Helmet & Buckler. I really like the them of this placement, as it suggests one of the various BG1 party members who don't appear in BG2 (such as Kagain) managed to escape Irenicus's ambush, carrying some items, and made their way to northern Tethyr before meeting their end. Or, alternatively, some Dwarven servant of Irenicus or the Shadow Thieves felt that these Dwarven items shouldn't be disgraced by being used by non-Dwarves, and took them and fled--either laying them to rest in the ground again himself, or being killed for betraying the Shadow Thieves or some such.
Krotan's Skullcrusher does not appear anywhere in BG2, AFAIK.