Complex sentence: They still lived in the mansion but were both lonely and miserable, which points to the fact that if Mikey had never joined LilyMu, then Mitsuki would have never met her crush and she would never be happy.
Split into simpler sentences: They still lived in the mansion and were both lonely and miserable. This could point to the fact that if Mikey had never joined LilyMu, then Mitsuki would have never met her crush and she would never be happy.