DC Fighter/mage should be able to use longbows. It's been awhile since I've played one too, but perhaps you need to regain your Fighter abilites to use them as Mages are unable to use them. It could also be insufficient strength; you need a strength score of 12 (I think) to use longbows and 15 to use composite longbows.
On the Horn of Valhalla thing; if you want the berserkers to be
really good, Haste them and watch the carnage.
The berserkers are among the best summons in the game, but to be frank, my solo Bard just reached Upper Dorns Keep and is beginning to see their limitations.