It does indeed sound like leprosy. I've never had/heard vomiting, being poisoned or other effects reduce endurance. Merely wandering the world map without visible negative effects shouldn't have any adverse effects either.
Enner is just about minimizing the time spent on GC5 with him alive. Blowing a scroll of detect material on enner beasr flesh, teleporting or running close to him with haste and blasting him with a few fireball blasts works perfectly fine. I rarely if ever have any trouble facing him with any character. Melee should be avoided without some heavily enchanted equipment and high agi/dex scores.
Actually, chao, according to the formula Planplan posted, the average damage of a weapon does increase by a flat +1 for each enchantment level. Instead of adding one to both min and max damages, every fourth enchantment increases the max damage by two while the min damage remains the same. Simple arithmetic mean.
There's no point in looking at the damage range variances for an item in a roguelike with hundreds of combat hits, it all averages out. It of course has an effect when looking at a single combat at a time in that luck plays a role, but a highly enchanted weapon is quite likely to go through quite a few turns of combat. Average damage is more significant.
Found some old pictures of mine, here's a character in valpurium equipment wielding a valpurium head of an enner beast killed by a valpurium golem. Natural int+wis scores as well from the dolphins, but those didn't count.