I too don't get the connection between events in the Shadowfell and the inn. If anything Selune's personal loss could well increase the resolve to aid her cleric.
I think crimsomrider has done an amazing job. There's lot of foreshadowing for this event. Nearly every book on you find in the game makes it clear that Sharrans do not get happy endings. Shar's clergy are feared, not loved.
Read the book in Ethel's tea house. Read the note on the raven. Listen to the story of the loyal Sharran shadow vestige in the central square.
Adding to the foreshadowing in print and voice there's lots of visual story telling as well. The first puzzle in the temple is solved when you extinguish all light. This foreshadows the extinguishing of the last light inn.
You're right that the death of one's loved one could inspire new hope and not the despair that Shar feeds on but I think despair is more likely; at least in the short term. As far as the mechanics - Gale answers this at the start and during his pixie circle scene. The shadow stuff is shadow weave magic it's difficult for either Shar or Selune to penetrate the murk.
I only play good shadowheart but as I read her story she is first impressed by the curse and then comes to doubt it. Is this what she wants? To see little girls who were good at climbing trees transformed into shadow monsters?
If she is pushed to be Justiciar she needs to put such thoughts out of mind. The candle in dark needs to but snuffed out. She might regret some of the lives lost but loss is something to embrace.
TL;DR Sharrans live sad lives.
