I think some people misunderstand the reason for Kurumi's jealousy, she's not just jealous because Rio is there with them (although she does seem to regret inviting her to sit down) but because Rio evidently knows a side of Ruriko that she has little to no access to. Ruriko speaks informal with her even though she still calls her own wife "Kurumi-san", she has obviously discussed romance with Rio too, which Ruriko and Kurumi haven't (for obvious reasons, as Ruriko is 100% seriously into Kurumi), etc. So I'd say its less that Kurumi believes Rio is going to snatch away her wife, but rather that she's shocked how a seeming stranger can have so much more intimate knowledge of her wife than she does
Aye, I also think this is the main reason. Hopefully we'll se some progress afterwards.