There is a way to do anything - the question is just the level of complexity. In your case, that shouldn't be too much of an issue, as both are relatively simple to do.
For displaying only one player, go to Window_MenuStatus and look for a line similar to this:
for i in 0..$game_party.size
Getting rid of that conditional and replacing all the i with 0 will effectively get rid of the other up-to-three members. From a gamedesign POV though, I don't know why you'd want to display the one character that isn't important for battling...
As for sending people in battle, I don't recall XP scripts too good to tell you the exact place, I think... one of the Battle classes should have a similar method to the one above though, in which this time, you change the '0' to '1', meaning it'll start adding actors from position 2 instead of position 1.
If you need anything done, ask for a move in the Script Request forums. In general, I think you want to do some crazy stuff that can be resolved easier and without messing with the systems too much... for example create a 'fake' Window_MenuStatus, showing the values of a database actor while you send all the characters in battle you have - as that character doesn't need to be in your party anymore.
