Keep the RTP resources private until official release?
and After the RTP is made will we be able to update it, as in Add too, and is credit given to the actual person that made the resource or?
Nah. We already allowed access to basically anyone as people often post their work here at CP as well and I already put up two downloads of the RTP. We're not going to keep it private.
If you use separate resources before the RTP was actually released, you should give credit to the makers. Technically you can give credit to all the makers instead of the RTP even if you use the RTP when it's released. We only offer the option to credit just the RTP so your game is not cluttered with the names of a bunch of people who's resources you're maybe not even using.