Was it inevitable and could the process have been reversed? Who knows. It happened. We cannot really prove a counterfactual.

We can, in a somewhat futile but admittedly entertaining exercise of hypotheticals, imagine what it would have taken for unification not to occur: Not only would the Kingdom of Piedmont-Sardinia need to have decided not to act in defense of pro-unification activists in the rest of Italy, but other independent Italian state would need to take the same decision. For governments of Italy to not take action, we need revolutionary fervor of the mid-19th century to not only to avoid gripping Italy, but pretty much also avoid the whole of Europe, and we also need there to be no rivalry or tension between France and the Austrian Empire in which the Italian peninsula's politicians could be caught up in. For that to happen, we need Napoleon to not have conquered half of Europe, and for that to happen we in turn need the French revolution not to have happened. The inflection points can go as far back as we want them to. See why this is futile?

I've written about the Italian unification process a few times. You might be interested in this somewhat recent answer of mine on the political beliefs and decisions made by Italy's first Prime Minister, who was a major leader in the unification process.