We were in the same position, a good amount of obc and wanted to book the restaurants and were unable to. We did upgrade to Premier once onboard, so we had 2 specialty restaurants included, so we wanted to take advantage of the package. We were a party of 7, so we went to Rudi's Catch the first night and had no problem getting a table. Then we went around to see if we could book another restaurant and did get a table at Sabatini's but never went. The maitre d wasn't great. He was the only negative for us on the cruise. We stopped by, he said he could give us a table at 5:00 on the day we were in St. Thomas, however, they put the reservation on San Juan day(we did the food tour so wouldn't be hungry enough for a big meal). When we stopped by to make the change he was very confrontational. At the end, he said he would give us whatever we wanted, but we canceled the reservation. We did stop by the Crown Grill and was told by maitre d to return the next day and he could get us a table and lo and behold he did. Giovanni, Alexa and assistant were all great in the Crown.
So, they will likely be able to accommodate you, but you may have to be flexible in dining times. We dined around 6 in both restaurants.
Also, you are able to use the credits in the casino. When you sign in the machine asks if you want to charge to your account, if you say yes, it asks how much, they transfer the funds to the machine you are on and you play. If you have obc, it just deletes from the amount.