I think we're agreeing in principle but coming at it from entirely different directions. I think you're completely right...but I also think I'm completely right and that our views are not in the least bit exclusive of one another.

Ianto speaking to Lisa after the brain transplant is rock bottom. There is nothing worse for him--it's final, everlasting proof (as are cyber-Lisa's last words) that she is dead and gone.

If you go along with the addiction metaphor, the one thing that saves an addict is the love of the others around him. Once he hits rock bottom, all those people who had to distance themselves from him come rushing back in to help him get back on his feet. They don't stop loving you, they never do, but sometimes they just have to step back and let things get worse before they get better. It takes a massive amount of courage, trust, and love to know when to step back, let somebody fall, and after they've plunged off that cliff, pick them up and hold them and hold their hand along the way.

And I think in the end Jack understands Ianto's reasons, or else he wouldn't have been allowed to stay.

I think that Jack understands, and also understands that there is nothing Ianto has done that can't be forgiven. Not forgotton, and he will have to atone, but he was forgiven almost immediately.