I think it's just maintaining continuity and trying to develop him. You play him too early and you could harm his development. Zizzo shouldn't be playing CB, but I have no problem with them holding off on playing Ndam. Especially a position like CB where so much of what makes a player good is in the mind.
I think they want to sign a CB, but just during the summer window. And I don't know for sure, but I would suspect it has something to do with the cap hit. Considering the fact that Baah is basically someone we can't count on anymore, I think we are probably looking to sign a starting caliber CB, so half the cap hit this season is going to make a big difference compared to the full hit.
That only works if the guy they are planning on signing will make more than Baah currently makes. An equal or less salary guy would come with no cap hit as a "replacement player". With that being said, I think you are right. The supply of high quality guys that would come before July window I think would be slim.