#Jun asked Shin to be serious about Noe and go out with her, which was, admittedly, very blunt of him. But the moment everything started to go wrong is when Shin brought forward the proposal. It was very unlikely that Jun would step back here, we all know what his personality is like, plus he must have prepared himself to agree to any of Jun's conditions for her sister's sake. Then Jun rode off, leaving Shin in total amazement. Was there any kind of contract signed? No, it was just a gentleman's agreement.
Jun didn't need to know whether Shin agreed to it, after all he was the one who came up with it, plus what kind of a man that doesn't have the bravery to do what he dares others to? So stop saying he ran away with it, OK? Then what? Like tons of opportunity for both Shin and Hiromi to tell Jun the joke had to end. But instead they just played along with it. Hiromi could always apologize and tell him she'd said it out of confusion just to shoo Tomoyo away or sth along those lines.
What's more:
+Jun did it for his sister. He knew about her feelings, so he'd agree to such a condition-->Now Noe's happy. (If there's anything you can call sacrifice, this would be it)
-Why did Shin do it? He was a clueless (and that's not an excuse) idiot who never thought it through.-->Now Hiromi's screwed up.
+Hiromi can break up with Jun any day without being heart-broken.
-After lying to himself, Shin went on to deceive Noe. Lovely. Or should I say, self-centered.
P.S: After Jun walked Hiromi home, she did think about him for a moment, this is probably a hint for future development. If everything works out for everybody in the end, then maybe Shin was not toying with Noe after all, and I'll be more than happy to forgive him for all his sins (ye I know I'm not God
)