I have been to Italy many times, big cities and small villages. My experience showed that the optimal focal length when there is a 24-105 or something similar. 24-70 is too short for me, but can still be used. Sometimes I did want to have something wider, so I started taking my 17-40 as well, and about 30 percent of the shots were done with that lens.
I tried to bring a longer telephoto with me, but quickly realized that I did not use them at all. 200mm, 300mm, let alone 400mm - all too long for narrow streets, or wide plazas, or inside cathedrals. I sometimes pulled them out to shoot architectural details, but it was not worth carrying that weight around.
Airport security is not anything special you should be concerned about. I came home to the States from Paris just two days ago, had two cameras in the bag (one mine, the other wife's), and nobody bothered me, either in Boston or in Paris. Once, I do not remember now in which European country, I was asked to open the bag and show the camera, but that's all.
Pickpocketing in tourist places in Europe, particularly Rome, Paris, Barcelona, etc., is something to be aware about, but not much of a concern. Just be aware of your surrounding, do not flash your money, don't put your wallet in the back pocket of your pants (I always carry it in my front pocket when there), do not put your camera bag on an empty chair in an outdoor cafe but rather put it on the floor next to you. Just exercise common sense and you will be fine.
Yes, I did have an experience of a pickpocketer trying to stick her hand in my wife's purse in Barcelona metro once. But it was only one such a case over more than twenty years of frequent trips to Europe.
Have fun there, Italy is a beautiful country.