Now that's a very intresting post. It also explains why Tuscany is so isolated on the culture map, because they are sort of in a corner especially when the papacy control the via Flaminia road.The Apennines are relatively easy to cross, being as a whole less high than the Alps; still, they are no slouch either, which is why the main route to cross them was the Roman "Via Flaminia" for centuries, going from Rome to Pesaro (before continuing north to Rimini). Loss of control over this route to the Langobards was the reason for the Papal State's dogged attempts to keep a hold on the so-called Byzantine Corridor, passing further north through Perugia, once Spoleto had fallen. It's also why I keep saying that a Perugia province would really be needed to represent the area, especially during the earliest start date, but that's not relevant to the thread.
TL;dr: if the realm has both Latium and Spoleto, then yes, it has little problems holding together.
It also explains why caesare borgia campaigned in romanga though again outside the game's timeframe.
I just realized how large the provinces in italy actually are, the region could really use a face lift.