Italy itinerary for 9/10 nights??

As you only have 9 or 10 nights, you might do well to limit yourselves to Florence and Rome after what I presume is a mandatory night in Milan due to late arrival. If you spend 4 in Florence and 5 in Rome, you can day trip to Siena from Florence and perhaps to Orvieto, Ostia Antica, or even Pompeii from Rome without having to change locations losing time. If 5 nights seems too long in Rome, consider that you won't do much beyond Mass, dinner, and a nice walk on Easter Sunday. The Amalfi Coast and Capri take a long time to get too. IMO too far for only two nights.

Not sure what time you land, but I dont know if you need to stay one night in Milan, unless you want to see something in Milan. The last train to Firenze SMN leaves from Milano Centrale at 8:15pm and the ride is only 1h 40 min long. So maybe you could get an extra night in Florence by going there on the day of arrival. 4 night in Florence will allow for a day trip somewhere in Tuscany.
I would not bother with Amalfi coast area for less than 3 nights, better if 4. If you decide to go, go straight from Florence to Amalfi coast then return to Rome, i.e. leave Rome for last, especially if you fly to Paris from Rome.

Two nights in Rome only equals one full day which it looks like will be occupied with the Easter Mass and everything else that goes along with that. Can I then assume that this is a return trip to Rome and that you have seen most of the usual tourist attractions on prior trips? We return to Rome over and over and have barley scratched the surface after several trips, mostly about one week at a time in Rome. April can be a great time to see Sorrento and the Amalfi coast, we loved it in April, but it can also be a wonderful time to delve deeper into Rome. After several trips, and one quite far south, I have yet to see Pompeii or Ercolano, and although we slept just across the bay from Vesuvius that is as close as I have been. I expect that we will finally get there on an upcoming trip to Puglia.

I'd absolutely recommend visiting Capri. The most beautiful place I have ever visited and there's lots to do. We did a tour to the Blue Grotto and it was the highlight of our trip. When you're in Florence, be sure to try the lasagna!

I would not try to fit in anymore; italy can be exhausting as there is so much to do. I am for staying 1 night in Milan and then leaving for Florence; we saw the Duomo, La Brera Gallery,the last Supper, and walked thru the Galleria, fashion district. Went to sleep early and took the 9:15am train to Florence. I usually don't recommend 1 night stays but when flying into the city, sometimes it is ok. Take another night in Florence, Rome and Paris and skip the south of Rome, that is what I would do if I had to do your trip.

Since you're obviously Catholic (going to Easter Mass, which is incredible), I'd plan to stay at the Alimandi Vaticano or the sister hotel Alimandi Tunisi which are both just across the street from the Vatican Museum. The Alimandi Vaticano has large family rooms that will accommodate four. I've been to Milan twice, and just never fell in love with it, so if you can fly INTO Venice maybe, and spend two nights, proceed to Florence and do a day trip to Siena from there by bus, and end with Rome, you'd avoid back tracking.
If you've already purchased your tickets and Milan is where you need to land, I'd agree with the other posters and get to Florence directly from there and proceed south. Personally, I'd leave Paris for another trip, since that city, itself is worth an entire week. Again, if the tickets have already been bought, and the cities are etched in stone, I loved the Hotel Eber Bars near the Eiffel Tower and Rue Cler. I took my granddaughter to Paris last year, and we went for the lighting of the Eiffel Tower every night, and she absolutely loved it! Be very careful with your I phones in Paris, as the thieves are masters at grabbing them!