Giangilberto from Milan here. Quick honest take on Cinque Terre first: late October is a gamble I've lost before, that coast in fall means shorter days, closed boats, and trails that shut after rain. It's not the 2.5 hours on paper, it's the changes and the "then what" if weather turns. With 11 days I'd save it for a spring trip and keep this one on the Rome–Florence axis, where October is glorious.
The town you're describing, historical, unique, on the train line between the two exists, and it's Orvieto. A volcanic rock rising out of the Umbrian countryside, one of Italy's great cathedral facades on top, Etruscan tunnels running underneath the whole town, and the station is right on the Rome–Florence line (about 1h20 from Rome). Two nights there is the sweet spot: evenings after the day-trippers leave are magic.
From Orvieto, if you want one more gem: Civita di Bagnoregio, a dying village on a pinnacle, reached by a footbridge over the canyon. Fair warning, no train for that last leg: it's a local bus from Orvieto, worth the small hassle.
And for your Rome days, the day trip Romans themselves do: the Castelli Romani, the little towns on the volcanic hills south of the city (Frascati, Castel Gandolfo on its crater lake). Regional train from Termini, half an hour, wine and porchetta country. It's where Rome goes to breathe on Sundays.
So my shape for your dates: Rome 4 → Orvieto 2 → Florence 3, plus Castelli from Rome. Zero backtracking, all on rails. Which sounds more like you two, the cliff-town drama or the wine hills? Happy to go deeper on either.
