There are lvl 8 & 9 spell scrolls in the game, IIRC, but you can't memorise them. Mods out there change that. I believe that, realistically, your mages would never reach sufficiently high levels to use those spells. Or at least that's what the devs thought at the time.
I'd add a Paladin, Fighter/Illusionist Gnome, or a Druid multi or dual class.
Crossbow instead of Bow: Bard already has bows, and there are some pretty good crossbows in the game. Feel free to spent another point in bows, and use the best of either, as they come up.
I distribute spells based on role: my Bard or Fighter/Mage gets all the buff spells, the main caster gets the offensive ones. Ofcourse, copies go to the one who doesn't have it already. Don't be afraid to spend money on scrolls.
Best weapon is hard to say, given the random nature of loot in IWD. Check this site's equipment database.
IIRC, the most powerful weapon is a Paladin-only Longsword.
A shield can just as well go on the Cleric/Ranger. Given the "half HP on level up" nature of multiclasses, I'm not even sure if the 19 CON comes into play. Even then, it would mean, by lvl 9, a 9 HP difference, at the maximum. The extra attack of the Ranger really helps.
Oh, btw, this extra attack also applies (wrongfully) to twohanded weapons. In this case, quarterstaffs. Not sure if there are any good ones.
Yes, give at least one point to slings.
Dualclass is generally speaking the same as multiclass, so go ahead. Be sure to take the Fighter side first, and go to lvl 3 (3 weapon stars in a weapon), 7 (extra half attack), 9 (5 starts in a weapon) or 13 (final half extra attack.) I consider 9 optimal in IWD.