Okay, I think I understand better your past, your concerns, and your point of view better. You are saying that your ex (perhaps inadvertently) created an NRE situation and a "unicorn privelage" situation. It seems that usually the original couple is the party guilty of exerting privelage, but it's certainly possible for the "unicorn" to exert the privelage and it looks like that's what you were subjected to.
Yes, the new person entering on a couple's relationship has less history with the couple than the couple has with each other. However, piling extra time/attention on the new person doesn't necessarily correct that problem. Perhaps more importantly, who says that it is a problem. So the couple has more history together. So what?
I am in an MFM V, and my two companions are legally married (since 1995), while I am married to neither of them. I won't lie to you; this was tough for me to take in the early days. They had their special jokes and intimacies with each other, while I was the "stranger" who was just awkwardly trying to fit in. But that was back in 2006. Years have gone by, and guess what? The "difference" between my history with them, and their history with each other, no longer matters. I have now accumulated enough of a history with both of them that I am satisfied, even if technically they still have more years with each other than with me.
So in other words, your ex's crusade to "level the playing field" probably could have been solved with a little patience and the passing of time. She didn't *need* to make up for lost time right away. She could have waited until the proportions and the percentages lined up more closely and didn't cause such a big issue anymore. Such was my experience (not saying I was a quick or easy learner).
So really, the biggest problem with Kuroi is that he didn't listen to your concerns about your ex in the past. He allowed himself to be swallowed up by NRE, and took his original family for granted. So, you are emotionally worried that he will do this again if he starts seeing someone new.
Does he recognize that he let NRE carry him too far with your ex? If he doesn't, that is probably your biggest obstacle now. He needs to have the courage to say, "Yes, I neglected my family for the sake of NRE and my ex." If he can admit those things, then, he can probably do better in the future.
I wouldn't be too quick to capitulate to his every want and interest just because, "Hey, we need to let each other be free." I've never heard a poly group say it was unreasonable to ask your partner to slow down for awhile. Just because that's not "perfectly comfortable" for him doesn't mean it's bad for him. If things went badly in the past and could go badly again, then you have every right to ask him to put a hiatus on his new-partner-seeking efforts.
Sure if you were telling him, "You can *never* have another poly partner again," that would be a little harsh, but if all we are talking about is a temporary slowing down and hiatus, he should be able to handle that. By the way, has *he* read the Poly Hell article? If not, he should, and you and he should discuss your feelings and opinions about it.
I think you need an agreement about privelage issues with *him* (nevermind what Polyamory.com thinks) before you and he can productively proceed with any further poly experiences. Polyamory.com certainly is obsessed about the "evil" of "couple privelage" ... and when couple privelage happens it is a problem. But now's not the time for you guys to be focused on what everyone else thinks, and on what everyone else has experienced. You need to be mutually focused on what has happened to *you* -- the two of you -- as a couple. If he's not ready to get onboard with that, then he's not ready to start dating new partners.
Such is my opinion about the situation, based on what I've read so far.
Love means never having to say, "Put down that meat cleaver!"