I would probably go ahead and just book it, since the cost is minimal to reserve what you want. Certainly some people have reported getting pins even though they have existing reservations, and then there are people like me who have gone often in the past five years and don't seem to get any pins. So it's hard to say.
In terms of receiving a pin or seeing a public discount on the Disney site - if you get one or there is a public discount, call WDW central reservations to see if you can apply it to your existing reservation. They may not be able to apply it if the exact resort and room category you want isn't available for use with the discount, but in that case you can alter your reservation if you want, since they'll tell you what you can book with the discount. Hope that helps!