A solo run with multiple participants seems like a nice idea, and challenging to boot! However, keep in mind that you can solo the vanilla game with any, and I literally mean
any character. Yes, even with a Beast Master. If you really want to make it challenging, you could make it a no deaths, no reloads solo game. Or install Tactics and Improved Battles. Now, that will spice things up.
At any rate, that's not why you came here, so I'll not bother you with those petty suggestions anymore. Instead, let us first analyse the characters you prefer the most.
First, the Blade. The jack-of-all-trades, master of none class in this game. I see this class as a slightly changed Fighter/Mage multiclass with the advantage of pick pocketing (buy all the great items immediately), traps, bard song, UAI ability and, also quite important, fast levelling. If you ever took Dalis along for the Shadows of Amn journey, you will probably know how powerful this class can get -if you manage it correctly, that is. Spells such as mirror image, stoneskin and PfMW get you through the entire game. Combine that with Offensive Spin and you get a powerful fighter (for that extra edge, try melf's minute meteors with OS and watch how quickly your enemies go down.) Longswords (Equalizer) and/or Katana's (CF) and/or Scimitars (Belm) are probably your choice of weapons. Basically, with a blade you can pretty much do anything you like. A high lore for identifying, spells to protect, disable and get nice XP boosts when adding them to your spellbook, Spins for that extra edge and HLA’s for possible cheesiness. Yes, you miss out on the real high level mage spells, but you barely need them anyway and if you really want them, you can buy them.
Second, the Swash/Mage and Swash/Cleric. Personally, I’m not a big fan of any dual classed character. I prefer to go all the way and get all the nifty abilities available to a certain class. That said, a Swash dualled to any of these two classes is still more than able to beat the game, although you’ll be very, very weak when you dual. As a mage, this can be just a minor bump in the road due to his ability to scribe spells in your spellbook, but as a cleric you’ll have more problems. The upside of this all, however, is that both the cleric and the mage class compliment the swashbuckler quite well. You can skip stealth because of sanctuary and invisibility spells and instead put them in open locks and find traps (or even skip the open locks ability in favour of pickpocket and instead use knock or the cleric’s strength enhancing spells). A swash can’t backstab, but he gets a THACO and damage bonus which might help both the classes you’re dualling to (thought not as much as when you would pick a fighter). Giving points to set traps will not be as useful as you might think; it will only net you a few weak traps. Setting traps is a task better suited to a high level thief. Weapons of choice would probably be Staves (Staff of Ram), or Flails (FoA) and Warhammers (Crom Faeyr!). But like you mentioned, this requires quite a bit of management and you really need to pay attention when you dual.
Lastly, there’s the Sorcerer. I don’t think this class needs much explanation; played well this is one of, if not the most powerful class in the game. Open to countless cheese possibilities and once you get project image you will never need to worry about having too few spells. I could go on for quite a few pages, but instead I’ll give you a few links which you might want to check out yourself:
First. Xyx’s spells guide. Basically, this covers the entire spell casting system of the game. Indispensable if you really want to learn how to use spells: [url="http://members.chello.nl/~j.vanthull/BG2SR/Main.htm"]Xyx's Spells Reference[/url]
Second: A thread discussing spell selection for a (solo) sorcerer which you'll find [url="http://www.gamebanshee.com/forums/baldurs-gate-ii-shadows-of-amn-9/suggestion-for-spell-selection-on-sorcerer-26939.html?highlight=Sorcerer"]here[/url]
So, it all comes down what your goal is: raw power (sorcerer), fun and almost unlimited options for situations (blade) or power and a bit of versatility but at the cost of a lot of management (Swash/Cleric or Swash/Mage). If you compare the melee capabilities, I think the blade has a slight edge as well. Yes, the swash get those extra thaco and to hit and damage bonuses, but unless you dual really late, I don’t see how much use it really is. Furthermore, it takes long before you’re really able to hit anything well because you’re going to dual. So, compared, the blade has the slight edge, especially because he levels much faster (but this point is also open to debate since you’re soloing anyway). Personally, I’m more of a fighting type and therefore I would go for the Blade. When I read your post on the other hand, I get the idea you really want those high level spells and thus the dualled swash might be your choice.
Hope this helped a bit. If not, I’ll keep an eye on this thread.
P.S.: I read the site you linked to in your post and I would advice
against using the Shapeshifter Rebalancing component from the Ease of Use mod. Trust me, it is horribly overpowered. You’re better off with the Shapeshifter fix from Refinements.
