Hi planningqueen -
So you're going to stick with the original plan so, at this point, thoughts about more or less time, here or there, are moot, right? As you mentioned not really being into museums + being on a budget, I might have reduced your time in London by a day. So many of that city's museums are free so are a great bargain. What do you intend to do/see there?
Yes, I am concerned about having just one full day in Paris as practically everything on your "goal" list for that city is outside. If your one full day pours rain.... Sure, with rain gear one can still sightsee but wet, cold and with weather-obscured viewing (Eiffel, Arc de Triomphe, Seine dusk cruise), you might wish you'd planned for a spare day. Granted, one can never plan around Mother Nature - you might have one dry, blue-sky day or two days or rain/fog/wind - but I've lost count of the number of times I was glad we'd built in flexibility...and needed that flexibility. It's good to be prepared for not everything going according to plan, including public/mass transit always running to schedule?
So what are your mother's interests? Other than the Paris list and shopping, what's on hers for your other locations?
Aside from London and Venice, I see you've already been to the other destinations on your list so probably don't need to tell you what the "sights" are for those. I will mention that I think you'll find that some things have changed in 11 years, specifically which attractions require advance tickets and how you go about getting them. Many tourist passes and what they do/don't cover have changed as well so you'll want to read up on that. As your trip isn't for another year, I'd hold off for awhile as procedures could change AGAIN if Europe is even more mobbed than it has been once open to most travelers.
Your 1/2 day at the Vatican: I'd definitely spend the money for an early entry small-group combo tour of museums + basilica. Hand's down, this is going to be the most efficient, least painful way to do both in that amount of time. An early entry tour will get you in and out of the Sistine before it's should-to-shoulder in there, and into St Peter's via a back passage reserved for tours, thus missing having to stand outside in the church's security queue. Most tours last 3.5 - 4 hours.
We are notorious over packers (a problem, I know) and huge shoppers so
trying to make those transfers as easy as possible.
Good that you're thinking ahead but your transfers are still going to be mighty painful if you're having to lug heavy bags through stations, in and out of trains, and up/down steep, narrow hotel stairways. Elevators are fewer rather than more in many Italian accommodations. Keep in mind as well that more stuff you have to pay attention to, the more distracted you might be to what's going on around you. As already mentioned, no one is going to know if you wear the same thing multiple days in a row so if you might wear it less than 3-4 times or so, leave it at home, and color coordinate what you do bring so that all tops go with all bottoms. Heck, in March what you're wearing will be under a coat much of the time! :O)
Shopping: good advice to keep it to a few smaller, quality items. All about lugging those bags, don'tcha know! Pictures/memories are the best souvenirs, IMHO. :O)
Florence... I loved Florence too and am an art geek so the Uffizi was a must-do. I'm guessing you've already been? Your mom, on the other hand, might not be into art enough to want to take that one on; same for the mighty Louvre and Pitti. Some smaller venues that are more easily digested might be more enjoyable for her? The excellent Borghese in Rome was mentioned before, and we enjoyed San Marco and the Bargello in Florence very much as well. There's some great art/architecture in many of the free churches in Rome, and some of the better ones in Florence as well that are either free or have economical entry fees.