SetCVar has a third parameter that is used to trigger the CVAR_UPDATE event and is the value passed as the first arg. So you would want to use
Code:
SetCVar('showArenaEnemyFrames', 0, 'SHOW_ARENA_ENEMY_FRAMES_TEXT')
to trigger that OnEvent handler (provided that Blizzard_ArenaUI is loaded).
I'm not sure why setting that CVar before Blizzard_ArenaUI is loaded isn't working though, unless some other code is unparenting ArenaEnemyFrame1-5 from ArenaEnemyFrames.