Oh dear ... honestly, I feel for you. I know how you go in circles when you feel for someone who is just unsure of what he feels for you. Especially in this situation with seemingly lots of NRE going around. This is ridiculously fast. You don't care for your own well being and try everything to just fix this situation and search for a glimpse of hope.
But you shouldn't let go of yourself. It doesn't sound like he will learn this any other way than the hard one. If he is set on exploring this relationship and doesn't want to listen to common sense and does everything what she is asking of him, including telling her that the decision has been made already while still lying to you about it (for whatever reason) … don't involve yourself for so long and suffer like that.
If you are able to, wait for him to come around eventually but separate yourself from this mess for now. I am getting angry just reading about it, but I guess that you are just hurting too much to feel something like anger. Try to think of the consequences, even if they seem to be far away. Please, don't just be understanding of him like your posts seem to imply that you are at the moment. Call him on his shit, that's just unacceptable behavior from my point of view. If he claims to be polyamorous, there should be enough feelings for you to make him think about you just for some seconds.
Sorry if this isn't helpful, I can't stand such behavior at all.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.