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Wow! Big read there, but it amused me [img]smile.gif[/img] Nice writing, Pirengle. [img]graemlins/thumbsup.gif[/img]

SPOILER (MAJOR as well!!)
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So far you've already done quite a lot of quests, not all in the way they were supposed to go (as you implicated the slaver quest was a little bit different). The other quests you've mentioned (Five Flagons Inn, Windspear Hills, Umar Hills) are all quests that can be done now (before you go off saving Imoen). For more spoilerish stuff read below.
To answer your questions:
- The staff is part of a quest in the sewers (involving a +3 Sword, which you might want to get). Go to the sewers to the place where you encountered Quallo, and head north by north east to start the quest. Further explore the sewers to find all the things necessary (watch the mouse pointer for things you can use/change in the area).
- Keldorn is a tank. As you said you already have a couple of tanks (Anomen is not a pure tank though, but can fight quite well). Minsc is a good tank, and Pirengle, your protagonist/ a fighter, is the second tank. IMHO don't get Keldorn, but stick to Jaheira. She has some nice spells (Summon Fire Elemental) which she gets quite early in the game.
If you DO want to get Keldorn, swap him for Minsc. That's what I'd do, as Keldorn is the only one capable of wielding one of the most powerful swords in the game; the Holy Avenger (which is a very good sword, trust me).

SUPER-SPOILER!!!
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You can also take on the quests in the Umar Hills (and all other quests you haven't done or encountered yet) when you've saved Imoen.