Yes you can join the ship at any of the ports, downside being that you obviously pay the full cruise fare as your cabin has been available for you from Day 1.
I don`t think so.
Easy cruise you can, or P & O and Cunard's World Cruise.
I picked up a leg of the QE2 world cruise from Singapore to Capetown.( I only paid for this part) I extended the holiday before the cruise and after the cruise by 3 days at each place and had a thoroughly good holida.
Yes, of course you can if they have cabins available. No proble.
You can get on an Easy Cruise ship at any of its ports, you have to arrange your own flights and everything though. I haven't been on the orange wonder so I don't know how good/bad it is.
They do not as a rule, but it is possible if there is space
of course there is always Easycruis.
You can join at a port on its itinerary on Celebrity cruises. Not sure about other cruise lines.