This is gonna sound goofy, but I think I might have finally understood one thing.
I was playing Aqua' story on lv 1 and as I got to Snow White's world, I was reminded that Aqua sees her in the glass coffin surrounded with flowers. You see, at to this point I have had one idea about the "not even memories are safe from the darkness" line. Some certain event has happened in a world, the darkness manipulates that event/memory in some way. Now that I was reminded that it was Aqua who saw Snow White in the coffin, maybe it's Aqua's personal memory that gets manipulated and twisted by the darkness. Aqua in reality saw Snow White as she was going trough the Dwarf Woodlands and now as she goes that place trough again, the darkness twist it to seems like that Ven is in that coffin.
It is the same case with Terra. You guys pointed out that Aqua and Terra met at the staircase when they were in the Castle of Dreams, so Terra manifests to that place from her own memory as an illusion. Maybe Aqua has realized that the darkness is playing tricks with her memories since she says that she is doing it again and the already mentioned memory line.
Probably something similar is going on with the mirror. It probably does reflect Aqua's inner fears, but once again the RoD possibly twists her memory, and the spirit of the mirror is transformed into mirror!Aqua. Mirror!Aqua uses a similar surround your foe with clones and shoot type of attack than the spirit of the mirror does, now only a bit modified with Aqua's attacks.
A long rambling about a topic that has already been talked about, but I was so glad that I am maybe finally grasping whats going on to a degree.