I really don't think we have to worry about a dead lesbian trope. The author has jumped forward multiple times and shown scenes where they're already together, and there are tons of illustrations of them when they're older.
Not to say there isn't going to be drama along the way, but I doubt there's cause for panic.
SJ is probably sick. Or Xiaonan confessed and she's freaking out. Or there was an accident and she's in the hospital, or she misunderstood something and is in panic mode, or....
Okay, so there's a lot of possibilities.