@stuckat1
I agree with all of your comments above, with the possible exception of the one having to do with display frequency. A video assist monitor that is being used for critical evaluation purposes must be able to lock onto the frequency of the incoming HDMI signal -- otherwise the viewer will see cadence errors -- or, more likely nothing at all.
In the US, even lower-end consumer monitors will lock onto 1080p / 23.976, 24, 59.94 and 60. As closed ecosystems, Android tablets have no such demand, they are purpose-built to display only their own graphics output -- and nothing else.
Even in a scenario where an external video capture solution allows for real time ingest of frequencies not fundamentally supported by a tablet's display, unless its signal processing capabilities are on the order of Teranex or Alchemist, cadence errors will significantly undermine its usefulness.
The hybrid monitors I linked are interesting products -- if you have the specific need. There are also a number of small, portable, HDMI-based "second screen" products that are intended to increase screen real estate for laptop users. However, none of these products are designed to support common camera output frequencies.
At the end of the day, it makes more sense to just purchase a dedicated video assist monitor. There are plenty to choose from -- some quite inexpensive. However, like everything else, you get what you pay for.