Sure thing ... . If I were to purchase a new release in the Magic Kingdom, I would think the Trading Post in Frontierland is the best place to purchase it and I would go there first.
As far as whether the release dates are concrete ... are we talking about a Limited Edition or an Open Edition pin? If it's a Limited Edition pin, then yes, the release date is pretty concrete ... unless there has been a problem with production and said release date has been pushed back (but it usually is announced - eventually - on the official site). If it's an Open Edition pin, the release dates are a bit more flexible - I've found Open Edition pins on the racks days-to-weeks before their scheduled release date.
The pins usually are put out in the morning when the stores first open. If it's a highly anticipated release, you may want to get there within the first few hours of park opening to guarantee getting a pin, but ... honestly ... lately, IMO, there have been very few pins that are in any danger of selling out within that first day, if not week, of release.
I don't know if there is a phone number to call to confirm a pin release, sorry. I've always gone with the information on the website, and if I had any questions, asked a CM at Disney.