She seems like she is basically saying
"Go ahead and date, I'm willing to go there. I will deal with whatever I feel once there. Including whether or not I feel like continuing to participate in this polyship with you -- that's on me. I won't block you. If it works out, ok. If it doesn't, not the end of the world."
That is and has always been her bag to hold. Her willingness to participate initially and her willingness to continue to participate is her decision. Not yours.
She's sounds pretty reasonable and realistic.
And you cannot predict/control every little thing. You yourself recognize that.
So go date who you want, kiss who you want, and before it gets to naked/sex time with the new person, give her the heads up so she's included and respected in the information loop so she can make her own sex health decisions.
Then she can decide if she's still in or if she wants to step off the polyship now. If she wants to step off -- you pause so she can step off with dignity and grace. Then you carry forward with the BF and the new person.
Hard to FEEL maybe. And that seems to be your trouble right now -- handling your own emotional
management on this. You seem to be over the hump though. Are you?
At this point it seems pretty straightforward in behavior
management so that all people are respected in the transition process.
I'm glad you guys have arrived at some sort of "work in progress agreement."