I think this is a perfect gift for your mother's birthday. She will love it (as will you). I wouldn't worry about the negative comments that you read. It seems like we as human beings do a real good job of complaining when things don't go right. But, when all is well - we never utter a sound. I think when something bad happens (at it does occasionally) these boards provide a place for people to vent their anger and/or frustration. Those of us who have enjoyed a perfect cruise vacation are just content to bask in the afterglow.
There may be some lines at the buffet, but other than the evening parade along the promenade, crowd control on the Mariner is kept to a minimum. And I believe that if you leave home with a general idea of how much you can comfortably spend on a daily basis, you can alleviate the money worries. Limit the amount of gambling, alcohol consumption, shore excursions and photography purchases you make to fit your allotted budget and there are no surprises. It sounds to me like you have the proper attitude to guarantee a great trip. Have a wonderful time and once again, ignore the disgruntled few and ENJOY!