If you wish to tip more or less than the suggested guidelines, contact the Front Desk to make any adjustments you think appropriate.
Yes,you are quite correct. You should contact the front desk and explain that you wish to tip directly and not have the payment added to the bill.You will probably be given a short form to fill in,but you should not have any problem.
One other thing; if you do tip directly please be generous as staff are not paid very much and they do rely on your gratutitys to make up their pay.
Yes you are correct. I have travelled on NCL many times and it is just a matter of asking them to remove gratuities from your account and then you are free to tip who you want to.