Venice/Florence/Rome and side trips itinerary questions

Re: shoes
Our Florence tour guide, an American who lives there, wore sneakers and Birkenstocks the entire time. I have foot problems too, so as much as I like to dress a bit more stylishly when in Italy, I draw the line at shoes.
Enjoy your trip!

Pompeii is a long day trip from Rome, and it's likely to be hot there. As someone who likes mosaics (some of which are still in place in Pompeii, rather than in museums in Naples or Rome), I preferred Pompeii (which I visited from Naples) to Ostia Antica, but the trip to Ostia Antica is a lot shorter--and cheaper.

For Pompeii you don't need to take a bus tour from Rome. You can take a Freccia train to Naples (buy in advance for good ticket prices, but be aware of the cancellation rules), then go downstairs at Napoli Centrale and get spur-of-the-moment tickets for the rattletrap Circumvesuviana to Pompeii Scavi. The travel time should be somewhat less than with a bus tour, and your time will be your own. You could (and should, because of the heat) get an early start so you have considerably more time in Pompeii if the heat doesn't wear you out.

There are usually guides hanging out near the entrance to Pompeii that you can hire on the spot, and Rick has an audio guide for Pompeii within his Rick Steves Audio Europe app. You could use one of those to get you started and then do some additional wandering if you want to. If the heat in Pompeii becomes too much, you could head back north and stop off in Naples for some pizza. Your ticket from Naples back to Rome will presumably be on a Freccia or at least an InterCity train, so you will have to take the specific train for which you are ticketed.

3-1/2 days is quite a short visit to Rome, so I think you should consider what you'll be giving up by swapping a full day in Rome for 3 or 4 hours at Pompeii. There are tons of worthwhile sights in Rome. If your group isn't too excited about museums or churches, that might be a reason to tilt toward Pompeii. It doesn't sound as if any part of your travel party is very young, elderly or mobility-challenged.

I wear lace-up, athletic shoes all over Europe. I hope no one is offended, but my feet insist on that kind of shoe. Which means my clothes sort of need to be casual, too. These are very similar to the shoes I've been wearing. And many of my slacks look like this--but bought on sale. On warm-weather trips to Italy I mostly wear lightweight, preferably crinkled cotton blouses, often purchased for 12 to 15 euros at street markets. Believe me, as long as you're covered up enough for the churches, no one will take notice.

The local trains between Florence and Pisa have no reserved seats and can't sell out, but getting tickets via app is the best way to avoid the long lines in Florence main station.

The high speed trains go from Rome to Naples in one hour and 12 minutes. You can hire a licensed guide outside Pompeii's main entrance.

Sorry, but nobody cares how tourists dress, with the relevant exception of those paid to enforce the rules to visit churches. Knees and shoulders must be fully covered, but do not worry to much about how you cover them. A paper shawl is enough. No local would feel offended by the way one single stranger among thousands of strangers dresses, most locals won't even see you.

There is no dress code in Restaurants, even the starred ones can't refuse service to anyone wearing clean clothes.

We would probably have to do this on a Sunday, the day before the Vatican day. Is Pompeii totally worth the day trip or would a trip to Ostia Antica be a good alternative?

Doing a day trip to Pompeii the day before doing the Vatican is likely to be quite tiring. The Vatican Museums and St Peters are intense sensory encounters. Fending off the fatigue of a day trip to Pompeii will not be helpful,
I think Ostia Antica would be a better choice for the time frame you have.

Several of the trains you will be taking are Regionale trains -- think bus on rails. No reserve seats and never sell out. Buy the tickets as you need just prior to departure or the day before. Train from Florence to Rome will be high speed and will have discount tickets available if purchased far enough in advance. However, discount tickets are heavily restricted so don't miss the train. No refund, no exchange.

Pompeii and Ostia Antica are not the same other than abandon cities - one buried in ash and another in mud. OA is a great day trip from Rome since it is full of trees and shade and easy to reach.

We never use our cell phones but others are totally dependent so it is your choice.