Well, #2 and 3 do a good job explaining how Nagisa and Ushio both died, but the entire concept of the IW is still as weird as hell. I thought it was just meant as a parallel or a metaphor for Tomoya and Nagisa. I didn't know that Kotomi's parents' idea was actually fact. I thought it was just an innocent speculation of the nature of the universe. Very, very bizarre.
I can accept the lights of happiness to a certain extent, but different dimensions connecting and what essentially amounts to time travel? I think Einstein just rolled over in his grave.
