Yes no problem, up to 4 people per cabin.
Yes provided its a 3 or 4 berth.
Yes, you will need to look on teh deck plans for the symbols which indicate whether cabin has extra upper berths or a sofa bed. First two in cabin pay full fare, with a reduction for 3rd or 4th person.
There are 2 single beds and then another 2 beds come down from the celling. I recently went with my husband, 13 year old son and 2 year old daughter and it was fine.
You are hardly in the cabin as there is so much to do on board.