They charge a 'corkage' fee of £5 from the onboard shop, its supposed to be per bottle but they usually just charge £5 extra for the whole purchase. The charge in La luna is around £5 i think, then you pay extra for certain dishes aswell. The Bistro is advertised on the website as under £20 per person.
You are not allowed to take alcohal on board, they take it from you and return it on your last night. Yes they do have a duty free shop for alcohal Not sure on the small cover charges, check the OV web page or the old OV answers. Have a good time we always do.
Go to reviews, ocean village, ov2 just above your question on the left of this page. There is lots of information from cruisers.