The command you're looking for, to turn off the health race bar, is this...
- /nui hud healthrace
This option toggles display of in-HUD health race bar on and off
You can see the complete list here:
http://www.wowinterface.com/forums/s...ad.php?t=21199 -- you can also use the in-game help... '/nui help' will show you the top level commands. '/nui help hud' will tell you the HUD commands and '/nui help all' will show you everything.
As for the mouseover... that's a layering issue that's been around forever. Hopefully I'll fix it once and for all in nUI6. In the meantime, just make sure the dead mob is not directly behind the loot window and you should be fine... you can just rotate the camera so the mob is out from behind the loot windows to fix that.