I don't think there's a "poly doctrine"; there is, at best, accepted wisdom -- and even that's likely to be challenged by someone, sometime.
If Pbn's poly partner were here, voicing his concerns on how to deal with his struggling mono girlfriend, it may well be that there'd people advising him to slow down. There would probably also be people saying that she's got a laundry list of impossible rules, go find someone else. That's the thing with forums, you find a great many POVs.
However, we're talking to Pbn, not her partner. We can't influence his behavior, only guess at it. We can't necessarily influence her behavior, either, but she's here looking for info/advice/ears.
They were in a poly situation; it didn't work out, and they both separated from the other partners. Now they're in a nominally mono situation (I believe), and she'd like it to stay that way. She is no longer identifying as poly, but as mono. He, however, is identifying as poly and so is likely, at some point in the future, to seek and find other partners, which makes her intensely uncomfortable and unhappy.
So, the rules. They come up, it seems, in almost every couple opening their marriage or relationship. On the flipside, breaking them seems to happen almost as much, and then trust gets lost and can be nearly impossible to get back. Then the relationship is likely to explode anyway.
I am not a fan of setting things up to fail -- and while I know that is never the intention, it does seem to be an almost inevitable result.
I like having paramour and metamour meet, but I don't think I'd tolerate it as hard-clad rule. Some people just don't want to meet their lover's other. It makes them uncomfortable. It may make them feel judged -- which can, at times, be the point of the meeting, to say "Is this person worthy of my beloved?"
I've read rules which state that anything other than kissing must be cleared with the primary first, often by stages. That definitely reeks of control-freakness -- and I say that with all good will, being one myself. That, however, isn't really a controllable factor. What happens when two people are alone together is going to happen, or not, and I don't see many stopping in the middle of heated passion to call/text their partner for permission, or even as a heads up, to move onto another stage. And then, boom, trust issues again.
You're not going to avoid those who want to lure the partner away by proclaiming primary partner status, anymore than a wedding ring guarantees monofidelity.
Et cetera. Pretty sure for every rule someone has made, exceptions have been made. Or they just get broken.
Rules are not going, I think, to make Pbn happy with the situation because she wants a mono relationship right now. Her boyfriend, though not currently with anyone else, doesn't. She doesn't want to consider a third person's feelings because she doesn't want there to be a third person, which almost guarantees there will be relationship sabotage even if only subconsciously.
It's a fairly major divide. If she wants to stay with the BF, she needs to reconcile with his polyness; I doubt he objects to her being mono. If she can't, then she can't, but then remaining in the relationship will only ultimately make both of them unhappy. She recognizes this, too, I think, which is making her miserable now.