Thanks for the question, @soulblight
We had to do a rules reversal regarding Eli Waters.
Going forward, this is the standing ruling, which will get included in the rulebook.
If a dude in a posse moves away from the location of a shootout, or moves home, that dude automatically leaves their posse.
To your question though, if Eli uses the trait and moves away from the shootout, he leaves the posse. If he was the only one there, the shootout will immediately end. Bear in mind, we do have this in the rulebook already under "Shootout/Make Plays"