Itinerary too full?

Depending on what time you arrive at each two night stay only gives you one full day at each destination.
Is that going to be enough time to see much? Not really. Have you been to any of these places before?
I’d cut out at least two or three destinations and take the one night in Rome and add it to the two you plan at the end of your trip.
Fly into Venice and out of Rome and leave France for another trip.

Which locations do you already have transportation that's locked in that can't be flexible - flight to Paris, flight to Venice, fly home from Rome?

What is the purpose of the trip and the cities you have chosen?

Seeing old favorite sites in a quick revisit or just checking new cities off a list by saying “I was there”.

Anytime I do a single city in less then 7 days I live to regret it.

16 nights, 8 locations way too much.
I’d drop 3 locations, give Paris Venice and Rome more time
Or maybe drop Amalfi area that gives you those 3 nights to add elsewhere
2 nights is just 1.5 days and that’s really only if you arrive in the am

As asked above- what is not changeable at this point? Flights, accommodations?

From one of your other posts, it seems you've only switched out San Gimignano from Florence, but the rest of your itinerary is the same. I think you're trying to see too many places in too little time, given the amount of time it takes just getting from one place to the next. Since the CT is likely to still be slammed by cruise day trippers, I'd lose that and add those days to a couple of your other places. Try to combine your Rome stays to the end.

I agree it's way too busy, you'll be spending about half your days moving from place to place -- packing, checking out, getting to the station or airport, riding or flying, getting to the next hotel, checking in, unpacking. (On second thought, not much point packing/unpacking, you might as well just live out of the suitcases.)

Without knowing what flights you might be committed to, I'd suggest sticking to Italy this time and giving Venice and Rome the days and nights you've planned for Paris and Nice. So fly into Venice and out of Rome, or vice versa.

But if this is next month you probably have the flights booked already. In that case, if you can bear to stay out of Paris this time, fly straight on to Venice. Anyway, you'll need to get going with reservations and plans for next month -- or are we talking about September 2020?

Remember your first and last days on any trip are wasted--in transport from and to the airport. The first day, you have to rest up from the change of time zones. Everytime you move from city to city, you also lose a day. On this itinerary, you lose too many days on trains. And note that distances in Europe are farther apart than most travelers realize--like the Amalfi Coast being 200 miles south of Rome.
I'm only good on trains for about 3-4 hours. Any longer and I'm flying most of the time. You cannot get to San Gimignano and Monterosso by train. Monterosso is west of Padua which is west of Venice.
Have you ever been to the cities you're wanting to visit? I agree about limiting your cities to three. You can take day trips out into the countryside. Paris is worthy of 4 days minimum. Florence is another 3 day city, and the Tuscan hillcountry is good for another couple of days. I agree about skipping the Cinque Terre as it's out of the way. Rome is another 4 day city as it is perhaps the most important city of them all to see.
You need to go back to the drawing board.

feeling a bit like we are going to be on the Amazing Race

Well, not really--- they are not aggressive in their planning

Paris- Venice-San G- CT-Rome is what I would suggest. Allocate your days according to your preferences.

Do not split up the time in Rome. If it is your departure point, do all your time there at the end

This Sept.?

I would leave out San Gimagnano-it wasn't as spectacular as I expected. There's much more to see in Rome(and Florence). I haven't been, but you can bike on the wall at Lucca, so that might be a better destination for you than CT and SG.

do this. Instead of listing the nights, list what you want to do in each city. What are your goals? I agree there are a couple places on your list that seem kind of "meh" but maybe you have a really good reason for wanting to go there? I have Karlsruhe on my next trip, and most people would never go there, but my wife's grandfather is from there, and she wants to see it, so we're going.

Also, instead of stopping at Rome for one night and then coming back, Go right to your next destination, and then spend longer in Rome. At that point you will be glad to settle for a few days in one place

Take a look at this itinerary, I've eliminated Nice, it's a 6 hour train ride from Paris wasting almost a full sightseeing day. Choose either Monterosso or Venice, you don't have time for both.

Keep in mind the times below do not include getting to the airport, train station, bus station or ferry terminal nor the wait times between transfers.

Day 1. Travel.

Day 2. Arrive Paris most likely jet lagged.

Day 3. Paris.

Day 4. Paris. (2 1/2 days).

Day 5. Fly Paris Orly(Ory)-Pisa (Psa) 1Hr40 min.
Train Pisa Centrale-Monterosso 2H02min.

Option: Fly Paris Orly to Venice (VCE) 1Hr25min.
Bus from VCE-Venice 30 min. Vaporetto to lodging 20-45min.

Day 6. Monterosso or Venice.

Day 7. Monterosso or Venice. (2 1/2 days).

Day 8. Monterosso-San Gimignano. Train & bus combo. 3Hr47m. Change Poggiobonsi.

Option: Venice-Florence Train 2Hr05min. Florence-San Gimignano 1H36m.

I personally would go from either Monterosso (2Hr28min) or Venice (2Hr05 min) to Florence and do a day trip to San Gimignano. Making an easier transition to the Amalfi Coast.

Day 9. Day trip San Gimignano. (1 1/2 days).

Day. 10. Florence- Salerno train 3Hr45min. Ferry Salerno-Amalfi 35m. Salerno-Positano 1Hr10min.

Day 11. Amalfi Coast.

Day 12. Amalfi Coast. (2 1/2 days).

Day 13. Amalfi-Salerno ferry 35min.
Train Salerno-Rome 2Hr54min

Day 14. Rome.

Day 15. Rome. (2 1/2 days).

Day 16. Travel home.

Hope this helps.

Dropping one city won't help much. Go back and plan a trip where you spend at least 3 days (four nights) in each location. Set aside a day for travel every time you change location.

I've been to every place you've listed except the Amalfi coast. There's just no point spending just a day in any place you've listed. My fear that so late in the game you've firmed up some plans. Tell us what is already booked and what can be cancelled.

It helps to know what is actually changeable without a cancellation fee. Are any flights booked?
I never do two night stays unless it is unavoidable. I'd drop half!

Thanks to all of you who have responded. Everything I have booked can be canceled except for the main flights - arriving in Paris (Sept 7) and departing from Rome (Sept 22). I am very open to canceling Nice and Venice.

Though at that time of year, I would personally be attracted to a week in Paris and a week in Rome, you have time to insert either Tuscany/Liguria or the Amalfi Coast in there. I'd really not try to do both.

...you'll be spending about half your days moving from place to place -- packing, checking out, getting to the station or airport, riding or flying, getting to the next hotel, checking in, unpacking.

Dick makes a very good point. It is amazing the amount of time you "lose" just relocating. For the amount of time you have available, four cities should be the max. Otherwise most of your memories will be a blur of train stations and airports. Don't mean to be Debbie Downer, but this may be a case where less is really more. Just one person's two cents worth.

arriving in Paris (Sept 7) and departing from Rome (Sept 22).

OK, you've got two solid weeks. If it were me, I'd spend one week in Paris and one week in Rome. This would be a wonderful trip that would give you enough time to soak in a little of the atmosphere of these two great cities.

I considered suggesting you insert a few days in the middle in either Tuscany or the Amalfi coast, but the logistics of adding another location would eat up a lot of time in travel.

The good news is you have a lot of flexibility. The bad news is that you are less than a month away from leaving and I fear you haven't made many (or any) reservations. You and your friend really need to have a serious conversation in the next day or two and then start making reservations.

Now that you've seen the logistics, consider what valadephia and DougMac have said, drop the Monterosso/ San Gimignano or Amalfi Coast portion of your trip. You'll save a lot of travel time and have a much more enjoyable trip.

Paris-Pisa-Monterosso-San Gimignano stays the same. Adding a day to Paris.

Bus,San Gimignano-Florence 1H36m.
Train, Florence-Rome 90 minutes.
Adding a day to Rome.

Other option.
Paris-Amalfi Coast-Rome.
Add a day to Paris.

Paris Orly (ORY)-Naples (NAP) fly. 2Hr10 min.
Look at Easyjet.
Shuttle to Amalfi Coast. 2Hr20min.
Add a day to Amalfi Coast.

Amalfi Coast-Salerno. Ferry 35m-1:10.
Train, Salerno-Rome 1Hr33min.
Add a day to Rome.

Travel times are reduced for either and you only have 3-4 lodgings to book. From the second itinerary, you have the ability for day trips in all locations.

Edited to add.

Paris, Nice, Venice, San Gimignano -Amalfi coast - Rome (skipping Monterosso) makes most sense to me but I'd fly form Paris to Nice
I'd keep Venice over San Gimignano and the CT. I'd add a day to Paris & Nice & Venice and not hit Rome twice but head to Amalfi then to Rome