First trip to Italy Need help with Itinerary

I would make some general comments but most are personal preference.

Naples is relatively safe as any large city but stay in Sorrento - far more scenic and interesting.

Venice is worth a couple of days. I would not take away one to go to Verona.

Don't understand why you would stay in Milan to do a day trip to Lake Como. Just stay two nights in Lake Como. Go to L Como from Florence. Or did I miss something?

If flying back to Rome to catch a connecting flight home on the same ticket, then that makes sense. Since you are not, take the train back to Rome. The train ticket if purchased in advance will be cheaper and quicker.

I don't think it is too much. If you haven't purchased your plane tickets then consider reversing the trip by flying into Venice and home from Rome. Called an open-jaw or multi-city tickets and often is cheaper than a RT ticket to Rome. Gain at least an additional day of sightseeing at no increase in cost.

When are you going? What are your interests? How are you traveling (train, car?). Personally, I think this is too much even without knowing the answers. And start thinking of nights, vs days. So on day 3 of Rome you are really traveling to next destination, yes?
Unless there is a hugely important reason to visit Lake Como, drop Milan and add to Venice. Two nights in Venice is better than 1 but I doubt you can add Verona. If interested in Verona, just go there vs Venice or Amalfi.
Absolutely NO REASON to fly from Venice to Rome. Train is just as, if not, easier and less of a hassle.
Really depends on what your interests are and how much time you want to spend traveling vs spending time in any location.
3 destinations total with day trips would be my recommendation.

It sounds like you already have your plane tickets and are committed to round-trip to Rome. More information would be helpful: What time do you land in Rome? When are you going? What are your main interests (art, history, food, wine, hiking . . . )?

Count nights, not days. 3 nights in a place gives you 2 full days of sightseeing. You'll use about 1/2 day each time you move: packing/unpacking, check in/out, getting to/from train stations, plus there's an orientation factor in each new place. Then decide if you want to travel first thing in the morning (and lose 1/2 day of sightseeing) or in the late afternoon/evening (picnic supper on the train and arrive after dark).

I guess you could take a late train from Florence to Milan, then spend the day at Como and return to Milan for a late train to Venice. It's just over 1/2 hour train ride from Como to Milan, then 2-1/2 hour ride to Venice. How long you need for the transfer depends on the train schedule - you have to allow enough time to retrieve your luggage from storage at the station. The current train schedule shows a train at 4.13 pm from Como that gets to Milan at 4.50, then a train to Venice at 5.15 arriving at 7.46 pm. If you're travelling May-July, that gives you some daylight to find your hotel. I don't recommend trying to find your way around Venice with luggage after dark - it's safe, but confusing.

Amalfi Coast/Naples - what do you want to see/do there? If you want to see the Amalfi Coast, don't choose Naples - it's too far for daytripping. I think it's too far away to fit into a 2 week trip that includes Lake Como, Verona and Venice. There are no trains to the AC towns. You have to take the train to Naples, then transfer to the local commuter train to Sorrento. From Sorrento you can day trip by bus to the AC. If you go "in season" you can also take the ferry (unless the seas are rough). However, in season, there can be long lines. Salerno may be a better base, with train service to Rome, and buses/ferries to the AC. Either way, you'll use up nearly a half day between Rome and the AC, and more between the AC and Florence.

If your flight lands early in the day, I'd recommend going on to Florence and starting there.

Your moving around too much.
Try this...

1. Travel

2. Arrive Rome early evening.
   Sleep Rome.

3. Rome.
   Sleep Rome.

4. Rome.
   Sleep Rome.

5. Rome. Late afternoon train to Naples. 1H10m. Campania
   Express to Sorrento. 1H07m
   Sleep Sorrento.

6. Amalfi coast by bus and ferry. http://www.positano.com
   
   Sleep Sorrento.

7. Capri
   Sleep Sorrento.

8. Pompei and Archeological Museum.
   Sleep Sorrento.

9. Curreri Viaggi bus to Naples airport. 1H20m. Fly Easyjet to Venice. 1H15m. Bus from VCE-Venice 30m.
   Sleep Venice.

10. Venice.

    Sleep Venice.

11. Venice. Sleep Venice.

12. Train Venice-Verona. 1H10m
    Train Verona-Florence. 1H30m.
    Sleep Florence.

13. Florence.
    Sleep Florence.

14. Florence. Tuscany tour.
    Sleep Florence.

15. Florence.
    Early evening train to Rome. 90 minutes.
    Sleep Rome.

16. Fly home.

Trains are very easy to use in Italy. Tickets if purchased in advance can save some money. For tickets use...
http://www.trenitalia.com/cms/v/index.jsp?vgnextoid=ee13721bdd69a110VgnVCM1000003f16f90aRCRD

As you can see, moving between cities can consume an easy half day. Checking in and out of lodging, transportation, and finding your way in a new area all take time.

Naples is a chaotic city. But safe as long as you are aware of your surroundings.

What time do you arrive in Rome on the 19th?

If you arrive early enough, take the train to Naples, check into a BnB or hotel near the train station.
Your arrival day is usually wasted so use it to reach your next destination. This will put all your Rome days together.

The next day do the Museum as early as possible, then train to Pompei Scavi, check your luggage at the site. When finished, travel to Sorrento.
Visit the area and travel back to Rome.

From Rome, after your partner leaves, take a train to Venice. Then on to Florence which is 90 minutes from Rome for your flight the next day.

You're going to have to trim a little fat from that itenerary. In my experience, sleeping in 4-5 cities for a two week trip is fine. More than that and you're spending most of your time traveling and not soaking up the culture. Maybe try just Rome, Florence, Cinque Terre, & Venice. Or whatever substitues you want. I love the Amalfi coast, but traveling there requires planning and time. I'm not big on Milan, unless you're into modern Italy, shopping and fashion. You're going to have a blast!