The question there becomes what counts as an "oversize"
scooter? Also, if I book a cabin that can accommodate a larger scooter, for example I book an accessible cabin, will they still deny boarding even though my cabin can accommodate the scooter that I have?
Of course the two cruise lines we prefer are Disney and
Royal Caribbean, Royal has staff that will take your scooter to a parking area when needed and bring it back if it won't fit in the cabin, which is also a reasonable approach. Disney can do this if you need to, but they take a lot longer to do so.
This also brings another point though, when we went on Symphony of the Seas, we had originally booked an accessible cabin, but DH put on the paperwork that the only reason he needed it was for the wider door and to have space to put his scooter, as neither of us needed the other accessible features. A bit before our cruise, our
travel agent called us and Royal Caribbean asked her if we would be willing to take a free upgrade to a suite that was not accessible, as their suites on that ship had a wider door and there would be room for the scooter in the room, the only thing we would miss out on was the door being automatic, but we got all of the advantages that a suite had to offer, including Coastal Kitchen, which turned out to be a huge benefit. They even gave us extra onboard credit for the inconvenience. Of course we accepted and had a nice suite for a week long cruise for our 16th anniversary. So would Carnival's policy also account for things like this? On a side note, always be honest on those forms and tell them what your needs are, we came to find out they offered us this deal because someone who really needed an accessible cabin with all of the features, such as roll in shower, etc. wanted to book and this allowed them to put the person in that cabin and allowed us a nice suite, everyone won in the end and was extremely happy with the solution.
But I wouldn't suggest ever leaving a scooter in a hallway or other public area, except maybe if you just needed to run into the room to get something. Of course if you have an accessible cabin on most ships the door will be powered anyway, making it easy to take the scooter in and out.
The other thing we found interesting on Royal is they have staff that can and will do minor repairs if needed. They do make it clear that the repairs are intended to be temporary to get you through the rest of the cruise and that you need to get proper repairs done as soon as possible and that they aren't responsible for additional damages, but if you agree, they will help you. Never had an issue while on a
Disney cruise, so I am not sure if Disney would do similar.