How hard is it to navigate Roma Termini?

It is noisy and busy you just need to take it slow look up at the board with all the listed trains.

You want the Leonardo Express which usually leaves from platform 23 or 24 every 10 to 15 minutes until 22:35.
There are several youtube videos on what it looks like and navigating
e.g. : https://www.youtube.com/watch?v=Az0DCWt58wc

Say no to anyone that approaches you asking if you need help.

Not hard - if you can navigate an international airport on your own, you can navigate Termini. Plenty of people to ask for help and they all speak English (if you ask nicely).

Are you planning to catch a flight the same day you travel from Naples?

If so, that may be the issue to address, as opposed to how easy it is to change trains at Termini.

Joe - why do you say that? We should have several hours between when the train should arrive and when the flight leaves. In fact, the reason we're considering this is because the airline just changed our flight time, and now we would only have 45 minutes between the two flights! So we're thinking of telling them to cancel the Naples-Rome leg of the flight and we'll take the train instead.

starting in a city distant from the Rome airport involves some risk. Normally with a few hours to spare between train and flight it works out fine. But if there is a delay on the track from Naples to Rome then you may miss your flight. Infrequent, sure, but it can happen. Depending on the fare you paid for the flight you may be able to pay a change fee to another flight (which might not be the same day) or worse just get a credit that you can apply against the much-more-expensive walk-up fare for the next open flight to the US. Personally I'd rather go to Rome the afternoon or evening before by train.

I'm a bit surprised the airline would change the flight and only allow a 45 minute connection since you're unlikely to make it. Many flights within Italy are on smaller jets (737 slze) that park out on the tarmac and then they send buses to bring passengers to the terminal. This easily takes 20+ minutes, then you'd need to get to the portion of the International Terminal area for flights to the US which is a bit of a hike, pass thru emigration, then another lengthy hike to the gate. Were both flights on a single ticket?

Was the original plan to fly NAP-FCO-USA? Was it on a single ticket or separate tickets?

I suspect it might have been a 2 ticket plan as it seems unlikely an airline would give you only 45 minutes to make a connection for what presumably is an international/intercontinental flight.

At the risk of being 'snoopy', what time does the flight depart FCO and where does that plane land? How much time you need at FCO is dependent on where that plane lands.

Blockquote
as it seems unlikely an airline would give you only 45 minutes to make a connection for what presumably is an international/intercontinental flight.

That's what you'd think, for sure. Surprised the hell out of us when Air Canada changed the flight that we had booked in its entirety through them was changed to this narrow window. We're pretty pissed about it, TBH.

if it is all on one ticket then they are obliged to get you home. inconvenient as it may be made. chuck the flight and you have no protection if there is an issue with the train. And if you do that, be absolutely certain that the airline is clear that while you will make your own way to the transatlantic flight you will indeed be on that flight. If you don't show for the first flight they are likely to cancel the entire itinerary with no recourse.

As mentioned be careful because if it is one ticket and you don't show for the first leg you can be cancelled from the following flights. Contact your airline and see what's going on.