Let me see if I can answer your questions correctly. ( My previous FPV flying has been with regular 2.4 gHz rc controllers such the Spektrum DX 18 and either 5.8 or 1.3 gHz video down links before I got my P3P...)
First, while it is true that both the FS Dominator V2 and the Dominator HD goggles both have an HDMI input, only the Dominator HD goggles support the resolution needed to use the Phantom P3 or Inspire HDMI output. The HeadPlays also work fine with the P3 and the Inspire.
The FatShark Dominator and Dominator HD goggles use a modular design that allows users to select which on-board video receiver that want to use, if any. Immersion RC offers several different video receivers which can be mounted in the FD goggles (only 1 at a time).
These video receivers cover the "FatShark" 5.8gHz band, the "Boscam" 5.8gHz band, and the US 1.3 and 2.4gHz bands.
The HeadPlays have an integrated 32 channel 5.8gHz video receiver that covers all of the Boscam and FatShark frequencies used in the US.
Many FPV flyers use a completely separate video receiver that is not mounted into their goggles. This allows for the use of dual antenna diversity reception, which is helpful when flying long distance FPV.
Ultimately, the receiver you should use is dependent on whether or not the receivers available are compatible with your video output.
I would also want to insure that there was adequate frequency separation between my control link and my video down link.
If your tablet supports an HDMI output you may just want to try using some HD goggles with the tablet first.
Hope this helps.
:harry