I have seen both age and weight limits on excursions They are set by the operator and yes I have seen them enforced. I was actually weighed before getting on a zip line tour.
OBC is On Board Credit you are not on board yet. The only thing it can be used for in advance is excursions and you wil be prompted to ask whether you want to use the OBC or not. We never book dinners in advance and never have a problem gettin a table after 7:30
We get some kind of discount on every cruise of at least 30%. For free cabins and free play we get balconies and between $250 and $600 in free play. These used to be up to $1,200 so the offers have gone down. We usually hit around 10,000 points each on average except the 2 occasions one or the other of us came down with Covid
There are several “super” premium liquors and I know the blue is definitely over the limit. Best to do is ask at a bar which brands fit under your plan