Is there any word on what the first DLC will be or roughly how long until it is released?
I also heard a rumor they were holding back the construction kit deliberately to bolster DLC sales.
Any truth to this? Basically, will the CK be out before or after the first DLC?

You heard a rumor from baseless speculation. It's been studied that tools increase longevity of interest, which boosts DLC sales, not the other way around.

The problem is they have to modify the tools to make them distributeable and usable in other environments. It's not easy.