I understand you. And pardon my hard-headedness, but based on your words in essence there is no need for a mode open for PvE, a private group can cope with it, when the player deliberately limits himself.There'd be no reason to remove anything from the game - it would be up to each player to decide whether they could cope with the limitations on their ability to shoot at anything that moves that choosing to play in an Open-PvE game mode would represent and up to each player to use their common sense when selecting activities.
There would also be no guarantee whatsoever of meeting a player who selected the opposing side in any CZ. If a player did meet an opposing player in a CZ in Open-PvE mode and couldn't cope with that then simplest solution would be "don't take those missions while playing in Open-PvE mode" or even simpler "don't play in Open-PvE mode".
Same with Powerplay. An Open-PvE game mode would be functionally very similar to one of the many PvE Private Groups, i.e. "don't engage in PvP, whatever you are doing in game". If players find that limitation difficult to understand or abide by then they'd very probably be better off playing in Open(-PvP) mode.
One of my old suggestions about openness is that a player with a notoriety higher than 1 can't get into an open game. It's as simple as that.