Hello jomni and welcome to the forums

As to your first question, unfortunately that feature (commanding a single corps in AI vs AI) is not yet implemented. Personnaly it is also one of the features that I am waiting anxiously, maybe in the next few patches it will be working.
As to your second question, you should Right Click + Ctrl the
Corps commander. In that way, the whole corps will move (although rarely, depending of the terrain, one unit maybe left behind and has to be moved separately).
Best regards