Well, in theory yes but in practice most ships sail full these days. If you are willing to pay for an upgrade, you would be better advised to try and get one now.
I am sure they would but there are very rarely cabins available to upgrade to as they tend to reduce last minute prices to ensure cabins are filled. This is particularly so with fly cruises as they need to fill the planes as well.
Yes but rarely available and if you upgrade on the ship it can be more than the UK rate which has a discount .