I dislike Itsume's betrayal of Rin's trust too but it shows how much she still has to grow as a person and is still in some ways the same scumbag we met at the beginning of the series. That being said Itsume's scumbag behavior has been the main driver of the plot up until now so I'm kind of hoping she keeps being a little terrible for the absurd situations it will lead to.
I get the poly stuff, its not for everyone and unfortunately there hasn't been a GL series that really does it right with the romance, even the few ones with a poly ending.
I would be so sad if this series ended soon, the Kagaya Inn crossover arc is coming and knowing Mochi its only going to get messier and crazier from here on...