Do enemies respawn in zion? because i dont see the point of returning when every creature is killed.

Yup.

In case you missed things, like: http://fallout.wikia.com/wiki/Desert_Ranger_combat_armor, for example.

creatures do, the tribals not so much depending on your actions

The creatures seem to respawn pretty fast. I'm not sure if that's only in certain spots or everywhere, yet....but sometimes it seems like they respawn every RL hour. Don't know what the actual rate is, I lose track of time...

Never mind the above. I think it was because I'd forgotten to uncheck one of the IWS (spawn mod) parts. I thought I'd unchecked them all but missed one, so it was probably doing something weird. Oh well, I had to restart anyway because despite reading about the fail quest issue I still managed to trigger it w/out realizing at first. I'm batting a 1000 today, sheesh. lol