OP, regarding the payment for specialty dining, I called NCL directly and reserved my dining, and this way the charge was not applied until I got onboard (and was thus able to apply any room credits to that charge). This is what I did about a month ago - not sure if anything changed in the last month since my experience!
Regarding gratuities - you heard correctly, they can not be removed before or during boarding, HOWEVER, if you have any issues or would like to adjust, go to Guest Services and ask for a "gratuity adjustment form" (that might not be the actual name of it, but it's what I was told to ask for). Once you get back home, fill it out and send it off to NCL with what you request for an adjustment. For example, we were in the Haven, and while we absolutely loved it (and for the price, with the included Haven restaurant, it was much less than concierge on
DCL!), our room steward really left something to be desired. Though we did ask to speak to someone about it when we went to Guest Services, we never were contacted to rectify the problems while we were onboard. Hence the gratuity adjustment, which I emailed in last week and received an email back from NCL that it will be taken care of in 14 days. I am not sure if that means 14 days in normal time, or 14 days of Island time (which might be 140 normal days, one just doesn't know if Island time is being followed

).
I really don't like that I can't adjust gratuities once onboard, but at least NCL will still allow it after-the-fact. At least, they did a couple weeks ago!