You need to either see all three interactions between Gin and Cassie, or just watch them interact thrice... not sure which and also don't really want to waste another 2 hours finding out.
I don't think so, I have all tags enabled in sanctum and haven't triggered it yet. I also have watched them interact thrice, with me snitching on Gin in two of them.
Edit: I finally got it! In order to see the third scene with Gin and Cassie, Gin needs to be in a maid costume.
You can trigger this by making him wear a maid costume during training. Afterwards, leave the town and sleep. If you get back and Gin is gone, don't go in the lake yet. Check the safehouse first to see if Cassie is there. If she's there, leave the town and enter again. Gin will still be in the lake so you just need to check the safehouse every time you enter. If Cassie is finally gone, go to the lake and there's a huge chance they're both there.
You need to progress his training until you have only 2 options to train him
If you haven't given the maid costume to Gin yet, the first option will allow you to, provided you have the full set.
Also, yes you can check the apple trees for Cassie, and the milk farm too just to be sure but running all over the place looking for other places she might be in is a bother so I just preferred to save before checking the lake and reloading if she's not there.