In the flashback of 198 we see CC save Marianne from committing suicide after she loses her parents and her house. This seems heartwarming at first, with CC taking pity on Marianne, but then you need to rememberCC's mentality at the time. She was taking advantage of Marianne's vulnerability since we all know she would've left Marianne alone at the first chance she got. Thankfully, as we all know, this does happen, but imagine if it had happened. Marianne would've been rescued by someone she thought she could trust only for that very person to not only leave her forever, but also take away Marianne's only hope of escape. Talk about cruel. Always friends? Sure they were.
A bit regarding the Grand Finale if Lelouch didn't come Back from the Dead. Right before the final battle he attempts to impregnate Shirley. 5 years later Susanna is the one having the baby. Meaning that if Lelouch hadn't come back from the dead he wouldn't have even had an heir to carry on his legacy.
We all know about Kasumi. Now, consider that for a brief moment in the timeline, there were two Kasumis; #1 Togabito Kasumi, and newborn Kasumi. Couldn't she have stepped in and stopped Tally from sending her younger self through time, saving herself from what she had been through? She either gets Ret Gone (she's a Death Seeker, so she wins), or even if anything bad happens at all, she'd consider it worth the risk. And Lelouch debunks Never the Selves Shall Meet when he fights Hollow Zero, so it doesn't apply. Hell, she could even serve has her own protector, Trunks style. Basically, she has absolutely nothing to lose by doing so.