Does the Nature Godlike lose their special ability if they drink a potion during combat  that raises Endurance above 50%

Yes.

 

I smack my Nature Ranger around until she's under 50% endurance and health before combat to keep her motivated, so the buff is always on unless she gets 1 shotted.

Yes.

 

I smack my Nature Ranger around until she's under 50% endurance and health before combat to keep her motivated, so the buff is always on unless she gets 1 shotted.

 

Sounds like a different kind of "game" and "play" there.