Save the fight vs. the demilich, Kangaxx until after the Viconia romance plays out and until Keldorn threatens to kill Viconia in a day...then when you go against the demilich and he imprisons everyone unprotected from it in your party (including Viconia and Keldorn), some kind of timer or whatever resets. Once your free them from the imprisonment, Keldorn will give the same talk he gave after making up with his wife.
If you just release Keldorn to try to do this, he will leave for good...well he did for me.

Not bad power gaming.

However I deal the demilich in chapter 2 and I don't have Freedom scroll just in that time. So, I'll skip it all.

Keldorn, goodbye forever!

Well, I certainly discover this while powergaming, not that I am opposed to it...I was preparing for Keldorn to paste the drow and I had a save when he said he was gonna do it so I was ready to go back and release the old boy if I couldn't rez the lucious one....regardless I saved the demilich because I couldn't handle it in chapter 2 (not much of a powergamer). My best weapon in chapter 2 was the daystar (Valygar) and the flail of ages (R/C protagonist). By the way the ranger/cleric dual class is the best tank I ever found...with good plate, shield and the dex you insist on when you roll the dice, AC is really good...like -10 to -12 if greedy with items like sewer cloak, elemental control ring...add the iron skins, defensive harmony, chant, bless....it can tank anything for a LONG time.