[Aerich]

Spiff,
I'd recommend *at least* finishing the Severed Hand before heading to HOW. There's a whole bunch of useful spells, armor, weapons and items in there; the place is loaded. For best results, I'd also do the first couple areas of Dorn's Deep as well. Since you're playing on Insane, you will really, really appreciate the elemental summoning scrolls you can buy there. I prefer to go through most of Lower Dorn's as well, as there are some awesome weapons there, and if you get them before HOW, you get more time to play with them =)

Your party looks like it will do well in HOW. It's a hack'n'slash extravaganza, and you have lots of fighter-types with good backup skills to help the combat along. The only concern I have would be with your fighter-that-will-be-dualed-to-cleric. I really don't see any point in gaining any fighter levels past 9 (some hold on until lvl 13 for the extra 1/2 attack/round, but I don't have the patience for that). At fighter lvl 9, you get the maximum benefit from warrior's HP and the HP bonus. After you reach 9th lvl, you only gain those piddling 3 HP/level. I would dual the fighter over to cleric now, then level him up in cleric enough to gain your fighter levels back for HOW. That gives you the advantage of having two extra-capable melee tanks with good priest spells (not counting the paladin or the fighter/druid, who also rock). You'll need those priest spells.

HOW doesn't let you in unless each character is lvl 9 or above. I'm not sure if that is adjusted if you play on insane. In any case, if you don't play through Lower Dorn's you may find yourself lowering the difficulty for HOW. A party of lvl 9 doing HOW on insane = big time ouch.