I know that P&O's Aurora is going on an eighteen night cruise this year, departing 9th November, and one of the ports is Malta, Tripoli has been taken out of the itinery due to a change of passport conditions by the Libyian authorities.
I have travelled quite a few times on Aurora at this time of year and can assure you that even with close to 2000 passengers on board, there are always lots of quiet places available.
If you want to check the P&O website www.pocruises.com you need to look for cruise R820.
If this isn't suitable I hope that you do find something else for your parents.
Best of luck ...Bob.
I think the two obvious cruiselines for your parents are Saga and Fred Olsen.
There maybe a problem find a company to insure them at a reasonable cost. I use Perry Gamble recommended by Victoria Travel who run this site. Currently we pay £110 each for cover for a year I am 60 and my wife 65 though. This gives us around 35 travelling days but no more without extra cost.