The next few months are going to be make-or-break for them. Carlo has experienced one of the worst moments that happen to most people, with the loss of his father. I really think Nicki's attitude is going to be what decides the future.
If Nicki keeps putting the pressure on him with the return to Tuscany, the constant work there - he's going to spring in the opposite direction and want to be in Positano. If she's constantly moaning whenever they are in Positano, but at the same time moaning constantly about work to be done in Tuscany, he's going to start seeing their new home as a burden and nothing more. Compared to the relative peace and quiet - and family - in Positano, I think it will push him back home.
If Nicki instead frames Tuscany as more of a peaceful sanctuary away from the troubles of living in Positano (such as the stairs, that they have more space, etc) - Carlo will be much more friendly about the idea of being there. If she doesn't fuss or whine about him going to Positano, being in Tuscany will be less of an issue for him mentally. He might even enjoy it more than being in Positano.
I think like most men, the more Carlo feels pushed towards one option, he'll run to the other. Right now he needs support and kindness and a bit of stability after losing the head of his family. Nicki is going to either bury her own grave here or actually patch things up.