My recollection is that Logan had political ambitions, and Geraldine was keen on "sponsoring" politicians, as she'd done with her sons and with Kevin Jamison. Logan was sort of a "distant conquest" for her. He didn't actually move in right away; that only occurred after he gained full custody of Jamey. He found himself having to hire an arsenal of babysitters, and Geraldine said, "Why don't you and Jamey just move in with me?" So he did.
I don't believe he really had a madonna/whore complex. Rather, he stumbled into the Raven Trap before he realized how "terrible" she was. Once he and Draper became closer friends, and he began spending time with Draper and April, he appreciated more that a man could have a decent partner. And then of course when Draper was presumed dead, Logan comforted April and fell in love with her. By then, he'd long since washed his hands of Charlotte's Web.
I wish I could recall more about Nadine Alexander Scott's initial exit in 1977. "French Fan" has been posting detailed summaries of the 1977 storylines, but there's no real mention of Ansel and Nadine moving to England. According to the summaries, the Tony Saxon criminal case ended, Ansel Scott collected a big check, learned he'd lost his position in New York, dallied with Raven, married Nadine (who was wise to the fact that Raven was flirting with Ansel). Then suddenly there was no more mention of Ansel and Nadine. Clearly they'd moved to London, but it's not specifically mentioned in the summaries. But there IS a detailed recap of a 1977 scene in which Nadine accuses Raven of "always going after her men", and Raven begrudgingly admits it's because Nadine "took my father away from me". Nadine basically explains to Raven that "your father never really wanted you, Raven. I didn't take him away from you; he just didn't want you. It's that simple", and that's a scene that I'd really like to see.