Really, I think I just want Izumi to date everyone.
She would probably dating no one by the endof the day thought.
or that it's unrealistic
Not unrealistic but not very common in wetsern society. I don't think polyamory is as easy as you made it sound like. It work require a lot of work of all the parts to not fall off into jealousy as polyamory is high maintenance. In term of manga, Polyamory require also high amount of work to not feel like a cheap way to end a love triangle because the author don't know how to end it. One of recent example i have is Luminous=Blue as the polyamory ending come unannounced and fell a bit flat because it come literally in the last pages as a way to not let any of the protagonist on the side.As for this one (stay on topicat least) Izumi is too much possesive to make a polyamory work in any way. I don't see any credible approach the author can made to make it work.
In my experience, there was a lot of work up front and then everything settled into a comfortable equilibrium. It absolutely does require tons of communication and honesty about one's feelings though.
I felt like there were hints in Luminous=Blue, but obviously I'm biased to look for them lol.
And I could imagine a scenario where Izumi is being this possessive due to bottling her feelings for too long and being backed into a corner. In that case, the possessiveness could be worked through. However, I agree that it's very unlikely that the author's going for that.
Realistically, I think Izumi's going to be used to push the main couple together, and then promptly yote from the story to be sad in the bad girls' corner with all of the other jilted friend characters.