[Aerich]

HoF mode (enemies with vastly multiplied hitpoints and damage-dealing capabilities) is very different from HoW (an expansion).

I wouldn't send a cleric into melee that often. I very much favour the ability to cast and tank combined in the same character - the F/M and F/C (or R/C) and F/D are some of the strongest class combinations as either multiclass or dual class. A cleric has a decent enough thaco, but the real kicker is the attacks per round - one just doesn't cut it; a high level cleric will usually be more effective casting spells than fighting in the front line.

Spells can be lost mid-casting quite easily - there's no concentration skill to be pumped up, and I'm not sure exactly how the game determines if you lose a spell (probably a saving throw of some kind). Generally, it's better to cast all the spells you're going to cast with a tanking/casting character before entering melee. The only exception to this is the F/M, as Mirror Image allows impervious casting from the front row.