Yep, it's ok to shift into D while on the fly and back again. Nothing different than normal shifting, only you are telling if where to stay rather than the tranny.
I run with the power button on all the time, save for being on the hiway. It seems to shift a lot more than I like while the cruise is on. Everyone has his/hers on preferences though and so, play around and see what you like.

Tone offers the Interceptor Transmission module. I use it and like the way it firms up the shifts and delays the torque converter to my preferences. There is a lot of talk about this subject so probably the best for you is to research it. Try searching the VMag for more info if you don't find enough here.