I'm in favour of the "hotkey" mode. The PC would probably know best how the party members felt. At least better than some random NPC (unless he's a potent diviner, of course).
Not in the same bit of scroll as the HP. I can display it in white text above their heads if a hotkey is pressed, but that seems kinda bleugh. I kinda like the idea of an NPC who tells you a bit about how your companions perceive you, but that seems more QP material than Virtue.
An extra non-joinable NPC, not unlike that Rom woman at Trademeet? For only five coins, she will tell you which member of your party plots against you, which is close to leaving, and which is ready to lick your feet...
Otherwise, is showing the happiness "value" above their heads together with HP technically possible?
I'm considering implementing a way for the player to find out how happy each NPC is. (This'll also show up bugs more easily...) This can't be done on the record screen, however. One suggestion is to have click-to-talk dialogue where the player can actually ask how the NPC is doing. This would, however, create some coding awkwardness with the Flirt Packs.