How do I kill the monster, after the first meeting with Irenicus, when Imoen tells me to lead him to her ....

Take the avatar to Imoen (lead him INTO the building where Imoen is) and fight him in front of her.

I had a similar problem--the guy simply WOULDN'T die, and Imoen just kept telling me to lead him to her. A simple Reload did the trick for me.

On a related note, I surrendered a point of CON. Once you're past Level 10 and have all your hitpoints, is there any point to having a good Constitution?

a higher constitution will allow you to have a higher chance of being successfully raised from the dead, but this is useless for you main character as the game ends when you die. Otherwise it would be useless.