Will the pad's display have hd resolution?
No. Similar res to standard 6-7" android tablet. 800x480 etc.
All the wii u pads were connected via wired connection to the wii u hardware at e3. We have yet to see hands on with a wireless connection streaming to the handset lag free.
I honestly don't think it would be possible to have more than one handset, even the tiniest modicum of lag introduced by multiple handsets would render them unplayable. Remember these are just touch screens there is no hardware / storage on the handset. The benefit of using 3DS as additional controllers is that the game could send data about the level to the 3DS and it does the computational overhead without introducing lag to the main experience.