What if the Expedition of the Thousand was able to take over Rome along with the Two Sicilies? Would that end the Roman question and unite all of Rome, including the Vatican, with Italy? How would that affect Holy See - Italian relations?
Holy See-Italian relations would be non-existent (which is even worse that in OTL). If Garibaldi takes Rome, he wouldn't be satisfied with leaving the Pope alone (like the Kingdom of Italy was). His soldiers would attack the Pope, attack the Cardinals, attack priests, ransack churches, and loot the Vatican. I'm not kidding here - just read Garibaldi's views on Catholicism, priests and the Pope (Pius IX specifically especially after 1848 and the fall of the "Roman Republic"). He thought they should all be abolished/destroyed. There was a reason Victor Emmanuel's government did not want Garibaldi to succeed. When the Kingdom of Italy took Rome it made assurances for the Pope's safety and independence to the point that no Italian govt. official could even enter the Vatican precincts for any reason without the Pope's approval - BY LAW. Do we think Garibaldi would care about such things? If any Cardinals or even the Pope himself was killed the condemnation of the Western World would hit Italy, even by governments like the US and Britain who were in favor of the fall of Temporal Power.
Needless to say, if Garibaldi took Rome and acted as he always said he would, the next conclave would NOT take place there and the next Pope would probably never enter Rome at all. Which would be another headache a United Italy (which wanted the Pope in Rome to prove how independent he was and they were not persecuting the Head of the Church) does not need.