I think the general understanding is that low season is May 1 to November 30, so you will be there in low season. Schedules for the ferries to Pinel from cul-de-sac are not 100% reliable - you can only be certain of last ferry time from Pinel back to cul-de-sac when you get the ferry over to Pinel.
Unless something weird is going on, your Royal Caribbean ship will have the same time as the local time on the island. There will be no difference in ship time and local time no matter the time of year.
If your ship is scheduled for 12:15 arrival, then you may be unable to get off until 12:45 or so - it may be a hour or more after you disembark before you are on Pinel. Your time on Pinel might be only 2 hours.
I would recommend you go to Mullet, as opposed to Pinel, on this trip. It will be easier and less complicated to go to Mullet. Go to Pinel when you have more time on island.