"I have been to Italy" is more common, but they are both correct. If you want to use the proposition "in", we usually add some extra information, and we often change "have been" to "had been" or "was" for this grammar structure. For example: "I was in Italy last summer." or "I was in Italy when I turned 21." or "I had been in Italy for 2 years after I graduated." The other phrase: "I have been to Italy." = This is just a statement that you have visited this place. Good luck!

Both are correct.

They're both correct.

I would say I have been TO Italy. In is closer to inside. I went to the store. I was in the store when the fire alarm went off and we all had to go outside.