The OP said "Our current configuration is a pretty straight V with my wife at the apex, but both the old flame and myself are open to it becoming a polyfi triad when we get to know (and hopefully love) each other better." That's what anyone should want to hear. That's "good poly". But no, people who are bitter and jaded by their own failed relationships have to come through with things like: "Just stop seeing a potential relationship with her as a possibility". Why should he stop seeing it as a possibility if it's something they have discussed and both may want? Because it offends you? Yes, people shouldn't form expectations from their metamours to start a sexual or romantic relationship with someone simply because they are metamours, but this isn't the case. This is something they both may want. That is something that develops organically. Discussing it doesn't invalidate how natural the progression was. What invalidates how organic a triad is is if one person always had the idea that poly people were obligated to sleep with metamours.
And this: " I can appreciate that you might not have the room but I honestly feel that bedroom sharing is the worst possible thing you can do.". Ok, it doesn't work for you, but um, who asked you to be in a relationship with any of these people? How do you know she didn't ask or doesn't prefer this arrangement? Because you don't, I guess.
I think that triads can be quite difficult to navigate; they have inherent challenges like any configuration. However, in the grand scheme of polyness, the OP sounds like they have a good chance of figuring this out. It might stay a vee, they might go on to a triad. Who knows? But he doesn't post any red flags for me. Not like the red flags obvious baggage and trust issues do, anyway.