Well I don't know which would be stronger. The Dr. C blend has other things to help the mouth so it might be better, but I would imagine that the straight white oak bark powder would be more potent in tightening the gums which you specifically want. I guess it's up to you. Perhaps you could buy the white oak bark powder to start with and then later buy the Dr. C blend?
I think you can brush your teeth with it but it would probably be good for you if you left it on overnight.
You could post a question on the use of white oak bark in the "Ask Barefoot Herbalist" and perhaps he could give you some instructions.