Best to do your own thing if it was me i would not chance any as you say you have to gage distances what if you cant get loo in tim.
Civitavecchia to rome by train is just over half hour, by coach it is over an hour (train much cheaper), Ville France to Monaco - about 20minutes by coach not far at all and very nice scenery en-route - one of my favourites.
Rome can have lots of traffic so perhaps train would be better but not sure if trains have toilets but at least you can get off. In France not far so you should be ok - traffic depends on what day you are in port.
The distance from Olbia airport to Porto Cervo Costa Smeralda by car is about 25 miles. If the ship excursion uses the direct route, most of the journey is on a main road. I think you would find it difficult to travel independently in Sardinia without a hire car.