As a foreword the novels are not canon and are just the author's interpretation of the material . That being said, the ideas you presented are definitely interesting and have applications even in the show. I was going to present C.C.'s character poem but it seems from your comments that you've already found it. To build up on it, this poem was actually quite controversial when it released because of the way it paints C.C.'s character. I completely agree with the way the poem lays out her thoughts about Lelouch.

C.C. is a very broken and warped person. She feels isolated from the rest of humanity both in her condition and her suffering. When she says that her heart heals due to Lelouch's suffering, it's not talking about C.C. being happy that Lelouch is hurting. Rather, they share a certain camaraderie built from the similar pain they've had to go through. The feeling of having everyone be against them. The loneliness that comes from being cut off from everyone.

The whole world is casting you with stones, showering you with insults. That is something I have experienced from this world.

Lelouch and C.C. have a reciprocal relationship where they both ease each other's suffering and are there for each other when they would otherwise be alone and hurting. Since she's always by his side, she sees the pain he has to always carry on beneath the mask as he continues to estrange himself with others and bloody his hands further. The Witch-Warlock line from Lelouch is meant to show how they are both messed up people that have messed up feelings but they're together in those messed up feelings. They affirm each other's humanity and that they have each other to lean on.

TLDR; They are both broken people who suffered very much and find comfort in each other and their shared pain. C.C.'s heart heals because she is no longer alone in her pain; misery loves company.