I think the PvP system in BB is the most fair for the non-invader player, and when I mean 'fair', I mean you have all the numbers to win and being an invder sucks most of the time.
The fact is that co-op in BB is a tradeoff, you make the game easier but you open yourself to be invaded. I think that's only fair, and means you only get invaded when you want to make the game easier (except in a couple of areas). Hell if you find the bell maiden, you can avoid getting yourself invaded and still enjoy the benefits of the co-op.
But even in the case of you getting invaded, you have to face an oppnent with 30% less HP plus if you have one or two more co-op partners, the chances are even more favorable to you. So you can't blame the invader to using all the tricks possible to even the odds.
At the same time dosn't mean you can't use similar tricks, to your advantage. Having one partner in a higher position using arcane and molotov, luring the invader when one of you hides and tries a surprise attack, or even just have the other dude constantly shooting the invader for a chance of a parry.
Is not a matter of even get gud, because 2vs1 means the other player must have really good skills, is a matter of not panicking and playing smart most of the time.
If your engaging in pvp, you should be fighting the person head on and not trying to get the enemies, which won't attack them, to do the work for them. Now, if the enemies attacked them as well (or if there was an item to trigger it like in DS's), I could consider that fair as that's a calculated risk, but as it stands, it's flat out cowardly and bitch-like using enemies to do your work for you.
Invader has 30% less HP, so he would need help to balance the fight. You also don't need to indulge yourself into it, in the sense, you just could wait on an area with no monsters and wait for more co-op partners. Of if you have another co-op partner already one could take the enemies and the other the invader. Again, you have all the tickets to the win, you just need how to use them,
When it comes to co-op invasions, if I'm playing with a friend and the person isn't being a dick trying to use enemies, I'll have my friend back off and fight them 1 on 1 fairly. Even though they annoy the shit out of me by invading, I'll still try to be fair about it whenever possible. When playing with a random person, I can't really guarantee that since I can't communicate with them. And if their doing the enemy bullshit, all bets are off and I'll tag-team them with ruthless fury. Regardless though, the person is willingly invading someone elses game (who might not want them there), so they should expect to be gang raped by 2 or 3 people from the start, so I feel no pity there.
And I never said they were constantly killing me. Yeah, they've got me before but it more of the fact that I'm tired of being annoyed with invasions while trying to progress.
"They use enemies to their advantage!, but is ok if I gank them with two other people"
That's the beauty of it, you can try to play fair, and as an invader I too try to play fair, but dosn't mean that an invader should follow any kind of samurai code or some similar shit. They are the invader, they are the bad guys so is ok if they act like one, in the same way is totally ok if you and your partner use you number superiority to kill him.
From the amount of people that seem to actively seek out pvp, I seriously doubt this would ever be a problem. As you said, it's been a staple of the series and plenty of people do enjoy it so it's very unlikely it would 'die' because an option was put it to turn it off. All it would do is help make the game more enjoyable for people like me who don't want to deal with it, which is probably an admittedly smaller % of players.
I can already tell you're a person who loves pvp so no argument I make about this is going to get through to you. Simply put, I just want to play my game with others and not deal with fighting other players. I know I'm not the only one out there that feels this way.
So you want the benefits of co-op but none of the inconvenients? too bad, that's part of the souls experience.