Wholeheartedly agree, Ken. It's the comic episodes being the norm that allows for the occasional serious episode to be such a nice contrast.
So, it appears that Abigail is officially gone. Can't say I blame her, but it'll be interesting to see how they handle Shaun having put himself on the line like that only to get rejected, and to see how they handle Juliet reacting to being chosen second. Obviously she'll be able to understand it from a police procedural perspective, but on a personal level that's gotta feel hard. Last year the show creators stated that seeing Shaun within a relationship was one side of a the same coin, and that Shaun might have to see what it's like to see Juliet in a serious relationship in the future. So her boyfriend from a few episodes ago will likely re-appear next season, especially since they planned another meeting one year later.
Loved all the Hitchcock references. The remote control crop duster was awesome!

As for who Mr. Yin is - if we're going with the premise that both Yang (in the photo with Shaun from 1989-ish) and Yin were friends of the family back then some serious things could go down when Yin returns. It's probably Yang's husband, but the why and the how should be interesting. Perhaps a former partner/associate of his father?