Two things I want to know:
[ol]What was the gift Umi gave Chizuru?
What did Umi and Chizuru talk about?[/ol]
For #2, I could see it go either way; Umi asked Chizuru out, or Umi told Chizuru that she needed to get serious with Kazuya. If the former, she turned him down, if the latter, she's mad that he's meddling.
Would Umi threaten Chizuru about revealing her being a rental girlfriend? I don't think so.
Getting tired of Kazuya's internal monologue. It doesn't seem like he's grown much.
And colour me skeptical that something big will happen in #180.
As far as Mami goes, she's going to tell Grandma that Chizuru is a rental girlfriend. The counter to this charge will be that Chizuru still has Gradma's engagement ring. Whether Chizuru owns up to it or not, the fact that she still has the ring is the counter to whatever Mami throws at her.