It's so nice to see other's posts. We personally plan on being back next year on the transatlantic and will still have mobility issues, so it's nice to see so much information.
To the OP, here's our info regarding walking/line standing. I did the 7 night med so I don't have some of your ports. For Venice, the info is from a separate trip I did a few years ago.
Villefranche
We did the Disney Monaco, Monte Carlo, Eze tour.
Mobility-wise, Monaco was "easy" because the coach bus parked in a structure, there's escalators up to the top level and there's about a 0.5 to 0.75 mile walk to the Palace Square. The walk is leisurely. You have time to slowly walk back and there are benches to sit on in the Palace Square.
Monte Carlo--- difficult for those with problems. Coach dropped at the bottom of a hill and there are steps and a steep incline (think San Francisco). Monte Carlo itself wasn't all that interesting, so this might be a good point to rest or just take in the marina area rather than walking up.
Eze -- difficult for those with problems, but lots of little shops and cafes to stop in during the free time here. The incline is steep but the walk is broken up (unlike Monte Carlo).
La Spezia
Florence was out of the question for us due to the description -- 2.5 miles of walking. We split up at this port. Some did Florence & Pisa, and we (the mobility issue people) did another excursion. The Florence/Pisa people said it was a LONG hard day, with tons of walking. They said that we (the mobility people) couldn't have done it without holding up the entire group.
If you parents are interested in Pisa, sign up for the Disney "Easy Pisa" excursion. This includes a ride to Pisa, a tram ride from the parking lot to the square (this cuts about a mile of walking). We tried to sign up for this on the ship but it was full.
Instead, we did Portofino & Santa Margherita. Highly recommended. Very little walking (just on sidewalks and near the marina). Lots of free time to sit, etc.
Civitavecchia
We hired a private van which dropped us very close to Trevi for pictures, then in front of the Roman Forum entrance.
At this entrance, there are elevators so they can skip walking down a steep hill to the bottom (this was helpful for us). The walk through the forum is about a mile from this point to the Colosseum exit, but you can stop along the way if you tour yourself.
Colosseum---- skip the LONG line by buying tix ahead of time. If you tour yourself, it's an easy walk from the entrance to the level where you can see the colosseum floor. We did the underground tour which was great.
Vatican--- I DO NOT recommend this for people with mobility issues. There is about 1.5 miles of walking through the museum itself and the hallways/galleries have no bench seating along the way or anywhere to truly rest. It was tiring for our group.
My father and I skipped this portion and did another tour. We also took cabs from the Colosseum to the Vatican/St. Peter's area to save from walking to the Metro. Fare was about 15 Euro.
St. Peter's--- unless you go through to St. Peter's after your Vatican tour, St. Peter's requires a security screening which has a huge line. If you do decide to visit St. Peter's, there are no seats/pews in the general area inside the church. There are seats/pews for those who would like to pray to the saints, but they do not allow just sitting/resting in those areas.
***If your cruise arrives on a Wednesday, there is the Papal Audience in the morning. Arrive later than 1pm in order to avoid the street closures/crowds so that your cab can drop you off closer.
Disney has a coach bus Panaranmic Rome tour for those with mobility problems. Essentially, you sit in a large coach and they drive you past all the sites and explain it while you're in the bus. We had considered this but decided Rome was too important to miss.
Disney also has group tours that do the sites in Rome. I don't recommend this since the coach buses cannot drop people off very close to the sites and there will be alot of walking (and standing around/waiting) involved with these group tours.
Naples
We hired a private driver and did Pompeii & Positano. Positano was easy to walk (lots of shops, cafes to stop in). Highly recommended.
Pompeii was very very difficult. Steep inclines, cobble stones. If your parents take it easy it might be do-able.
I would consider going to Herculaneum instead. There is a TR on here where the poster went to Herculaneum instead and it is less crowded and the paths look less steep.
Venice
Buy skip the line tickets for the Basilica of San Marco.
Buy skip the line tickets for Doge's Palace. I recommend the "Secret Itineraries" tour which is a separate tour that comes with tickets. You take the tour, then you can go back in through the Palace at your own speed. There's no real benches to sit on during the tour, but it's not long and it's fascinating.
Good luck, and have fun