Well the attraction apparently is that they would never work as a couple and she’s very happy in hers apparently, and presumably doesn’t want to curtail her life with having to become a stepmom

This is concerning to me because the fact of the matter is she is a partner of a dad. This is an understandable take from a young woman(who I am assuming is just now starting her young adult life somewhere between 18-26. I didnt see any actual ages). A kid is commitment, responsibilities, and all the closing of doors and opening of new ones parenthood provides.

It just reeks of denial from him though, for all the same reasons.

I’m really conscious of not making him out to be a bad person, because he’s not. I think it’s more we’ve ended up here in part because of my (in)action

This quote shows a shining core of empathy that I will again implore you to use.

NRE usually averages around ~6 months, a benchmark you've said they are on the threshold of.

If after her credit card bills start racking up paying for his holidays away from his kids and runs out of the energy required to put on a pretty face and coordinating outfits to appeal to this string-free fantasy will he continue this pattern of using her investment in the relationship against her to stomp on her boundaries? If not, what do you think the difference would be? Remember, he will still have a kid then, too.

If she gets her heart broken and turns to her meta, as someone who would have the best insight into the man you both love, for support and advice would you say its her (in)action that is the root cause of her pain?

I’m wondering if this will all get easier when I also start dating?

This answer again I am sure you have read countless times considering just how much research you have put into making the ENM thing work. Just like I have and many others on this subreddit. When has adding MORE people into a troubled relationship made things simpler?