I'm sorry I didn't get back to your thread sooner - that is an absolutely amazing story!
Congratulations on making such a smooth transition. My position is similar in many ways to your wife's. I live with my husband and my boyfriend. We have had a few wrinkles regarding boundaries which have actually become much easier to iron out as they come up the longer that we have been together - as each person learns to see and understand the position of the other(s) - so I think that the fact that you all have done so well so far bodes very well.
I do think that developing a strong friendship with your metamour (your wife's boyfriend) is key, especially since you are "cohabitating" to some degree. If you end up caring for and respecting each other for your own sakes (not just for your wife's sake) then that makes it even easier to consider the other's reactions. (Dude and MrS had the advantage of already being best friends before I even met Dude.)
I am very impressed by the path that you have taken, and wish you, your wife, (and her boyfriend) all the best. There will be bumps in the road (we are still running into them two years in) - feel free to use this forum as a sounding board. (or PM me if you want a female "hinge's" viewpoint - not that I am necessarily anything like your wife).
Do your wife (and her boyfriend) have any poly resources they are utilizing (like this forum)? Sometimes people who are caught up in NRE (new relationship energy) can get so caught up in the "new shiny" thing that they can lose their perspective - forums, like this one, can provide some insight as to how to navigate some of the milestones.
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (23+ yrs)
Dude: hetero poly male, live-in boyfriend (4+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe