I've turned off AI (too many characters standing around in Undersigil, for instance), but I can't get Daakon to unleash some of his most powerful spells without his getting gravely injured or killed. Once I tell a group to attack, are they meant to continue fighting while I pull a spell caster behind them to work his magic? I can't do the one without stopping the other. Somebody worked out a good fight/magic combo?

What do you mean can't do one without stopping the other? First select everyone so that you don't have to direct those who don't use spells manually.
Then just click on the spellcasters (let's say Dak'kon for example), click on another part of map a bit further off, wait till he gets there and start casting with him while the others fight. Or just pick Dak'kon as soon as you see an enemy and target it. He'll stop early enough that you fighters can get in front...

I guess I'm struggling (and failing) to collect fighters and send them to attack; in *Baldur's Gate* You can hold down <ctrl>, hit the number keys of the fighters, and have them attack. Planescape won't let you do that; the number keys get you odd combos like one-eyed jacks, kings with the axe, fives, sevens, and nines. Or like that. But good to know that if I can catch them all, I can set fighters to fighting and then concentrate on Dak'kon. Much obliged.
O.

I really can't understand something here...

You can edit anything in quickkeys, and to attack someone default quickkey is "A".

What is better in BG I can't see...

oyster, you should explore the game interface a bit more. It's explained in the manual. There's a key to select all party members, so you should never have any trouble catching them all over the screen.