I believe the idea of sending Nick down was done for numerous reasons -
He was struggling so bad that he was not going to work his way out of it here - they gave him an extra 4-5 weeks to do that and nothing was working
If he needed to make mechanical adjustments it is much easier to make them out of the glare of the spotlight and pressure of the major leagues -
If it's not mechanical and he needs to get his head on straight and get some confidence there are few places in the world better to do that than the friendly confines of Tucson/PCL
it also allowed them to get Grandal up which I believe as much as anything was just to see how far away he is from being major league ready - Contrary to what you see posted on this board there are some smart baseball people in the organization and they know not to get too high on his early success - but that said - if he can show he is ready now or by next year they can develop their plans...
I don't know how Byrnes feels about a catching tandem - KT and Hoyer showed a willingness to do it - but that may have been just because Nick wasn't ready and they didn't have another catcher in the system ready and they believed Nick could be the guy...
Unless Grandal becomes Buster Posey I suspect they will do some form of tandem - Baker appears to be an ok hitter but I don't think much of his defensive skills -
Maybe the idea is let Nick rehabilitate his offense - gain some confidence and IF Grandal is showing well to trade Nick because he has some value - get something worthwhile in return and go with Grandal as the main guy and Baker as the back-up -
Nick might flourish in a more hitter friendly park/division -