I don't know the specific answers to your questions. But, Rome Hop on, hop off is not recommended here, unlike other cities. First, due to small streets in many areas, the bus can't get close in many places. Secondly, the buses are usually very busy, and it has been reported to be difficult to get back on the bus when needed. Third, many of the most popular sights are is pretty close proximity, so they can be walked easily.
We do hop on hop off in many cities. But from what I have seen in Rome, and what has been reported here, I wouldn't do it in Rome. Perhaps a better idea is to take Celebrity's "Rome on your own" bus, where they will drop you need the major sites, and you can walk most of them.