Train Florence to airport (FCO)

Florence to FCO in late May for 12:55 pm flight at end of trip

Why not say the exact day so people can tell you whether not you are looking too far in advance which it sounds like.

Moreover, don't use Omio. They are a 3rd party and don't own or operate the trains in Italy.
To avoid any unpleasant surprises you should only use trenitalia.com and Italotren.com; these are the only two companies that make the schedule for the trains they operate.
It is best to go to Rome the night before the flight if you can't afford to miss it.

ItaliaRail and Omnio are third party sites (based on my knowledge, may be incorrect). Forget them, they sell only select routes.

Italo is an independent operator, offering limited high speed routes in Italy. They do run routes from Florence to Rome Termini station, but not to the airport.

The site you are missing, and the most important one is Trenitalia, https://www.trenitalia.com/en.html the national train operator, that goes most everywhere.

If Italo shows a train going to the airport, then they are likely selling you a ticket to Roma Termini, then including a ticket on the Leonardo Express operated by Trenitalia, to get to the airport. Trenitalia will have many more options, most all will include a change at Termini as well. Very few trains go directly to FCO.

As for a back-up plan, sure, a car service would be an expensive back-up, if you are worried about risk, stay the night in Rome. The further away from the airport you get, the bigger the risk.

Hi. Use trenitalia website, do not use the other sites as they are 3rd party sellers. Use Firenze SMN and Fiumicino Aeroporto as stations. For the random date I picked, the 7:05 - 9:22 train is listed but not for sale; it likely will become for sale later; otherwise there are trains at 6:00 am or 7:14 am. It is very early to buy tix now; these trains have large capacity; you may want to wait until about 30 days before travel and then check again. Have a great trip!
p.s. - good comments by others about possible risk. There is about a 98% chance that the trains will run on time that day and you will make your flight fine. I have done it several times, but only when I had plane tickets that were changeable and I was aware of an alternate flight. Up to you whether you plan for / OK with 2% chance of missing flight. A last minute car service is not a back up plan; the train is high speed and by the time you found out there was a train problem, a car could not get you there in time. The only true back up plan is to be OK trying to change your flight or buy a new flight at the last minute. .

Q2: Direct train (no transfer): On Highspeedtrains.com, it shows a
train departing SMN at 7:05 arriving at FCO at 9:22 (the TrenItalia
Bullet, train #9591). This train does not come up in the ItaliaRail
search. Any guidance?

The train that makes this no-changes run - either direction - is showing as not purchasable/available for upcoming months on the Trenitalia website (and yes, THAT is the website you should be using). There's been some discussion around that in some other threads so you should look at options involving a change.
https://community.ricksteves.com/travel-forum/italy/nonstop-train-from-rome-airport-to-florence

As above, Italo doesn't service FCO.

As well, there's no way I'd personally want to be in Florence on the morning of a midday international flight from FCO. I'm with Bob in that should there be a rail issue (strike, unplanned track problem etc), a car service isn't a good backup plan plus it would be a very expensive ride from Florence.

Do you understand that Roma Termini has a dedicated airport train that is walk-up service? You are not looking for a full booking from Florence.

I agree that waking up in Florence for a 12:55 non-refundable flight home is quite reckless. Even if you were in Germany, their fine train service is not as good as it used to be. Italy has never been renowned for punctuality. You are required to check-in three hours before departure. At this busy time of year, there could be long security and check-in lines.

Having taken umpteen trains in Italy in many trips there, there is no way I would advise anyone to travel , that morning, from a city other than Rome to get a noon-ish flight at FCO.
Too many variables.
You need to be at the desk at FCO checking in by 09:55…..you’d need to leave Florence before it’s even light out.
If you decide to go ahead, the only other thing you need to be sure of is having enough in your bank account to pay for a new one way flight home should you miss your booked flight.

Spend less money and be in Rome the night before your flight.
Less stressful all round.

As others have suggested, only look at the Trenitalia or Italo websites. To get a better idea of what will be available in May, look at the date next week that's the same day of the week as your May date. Search on Firenze SMN to Roma Termini to see the high-speed trains running on the route at your intended time - the schedules won't change that much. The train from Termini to FCO is a shuttle and you would just buy a walkup fare on the travel date.

It's way too early to buy a ticket on that route - wait until at least later in April. But take heed of the concerns voiced above - this is a high-risk plan.