Welcome to the forum. :O)
Italy is a big place.
Europe is an even bigger place.
Many of us here travel independently because it gives us the freedom to arrange our own itineraries according to our individual interests; to go where we want to, when we want to, and for the amount of days/weeks we have available or can afford. It isn't necessarily everyone's style as it involves a LOT of research to choose and book hotels, buy advance attraction tickets for those which mandate reservations, figure out public transit systems, and get a handle on some of the cultural differences we'll likely encounter.
Some travelers - especially 1st-timers to the continent - prefer escorted group tours that handle a lot of the details for them. They provide the hotels, some of the meals, some guided tours + tickets to some of the attractions, transport from place-to-place, and general support. That doesn't mean just signing up, and showing up, without any prior reading about what they'll be seeing and experiencing; time with a guidebook(s) is still a requirement to get the most of the tour.
Looking at your questions, which are much too broad to be tackled when we know virtually nothing about you, you might look at some of the RS tours offered on this site and see if any appeal to your interests and budget? For instance, here's a good intro to the "Holy Trinity" of most-visited Italian cities.
https://www.ricksteves.com/tours/italy/venice-florence-rome
Willing to do a bit more of the work yourselves but have your hotels and place-to-place transport handled for you plus some general support? You might be interested in a "My Way" tour. You do need to arrange your own daily sightseeing tickets, in-city transport (if needed) and most meals yourselves.
https://www.ricksteves.com/tours/italy/my-way-italy
So think about how much you are or are not willing and/or comfortable to take on yourselves? Speaking only for Italy, much of the country is not difficult to travel, and basic-to-fluent English is not uncommon in the tourism industry so no, you don't need American ex-pat guides! As you've expressed specific interest in "The Boot", I'd echo the advice above to limit this trip only to this country so as not to become overwhelmed. It's a fascinating place one could take a lifetime to digest! Same with other European countries.
Yes, do involve your niece in the plan, This is an opportunity for her to learn and experience what it takes to travel efficiently/effectively; research, booking, packing, etc. Honestly, if she's not showing a reasonable level of interest and willingness to get involved, I'd put the trip on the back burner. As well, the more you can tell us about yourselves, the better people can help you.