Q2: There are safety boxes at the reception counter. You can put your valuables there.
Q3: It depends on which activities you want to take and the timing. My advice is to book early on Saturdays and Sundays. My experience is that I book a mangrove tour at 9am and depart at 10am. But spa treatment is already full for the whole day.
Q4: Yes, it is more or less the same arrangements as if you go to Macau. If you have a large suitcase, you can put it to the luggage counter before boarding. The porters will handle the luggages for on/off the ferry.
Hope that other BK members will give you answers for Q1 and Q5.
For question 1, I bought the air tickets and ferry tickets and book the hotel separately. I found this was the cheapest way. You may need to check the package price first and compare.
For question 5, there are not much special thing to eat in Bintan. Most of the time eat at the hotel. If live in Niwana may go to their Seafood resturant. Hotel provide the shuttle bus to that resturant.