the Phantom Slasher seems to be the most lethal and effective killer, which could point to the idea that a Shadow Sword's power increases with the amount of Auras it absorbes. And we know Mimic's been around for a long, long time..
Yes.. Mimic has been for so long and yet the phantom slasher seems to be relatively new in comparison at the beginning of episode 1. Something has evidently triggered it to start manifesting itself more often.. No idea what, but it could be increased power in the sword.
As for Trazuin's mother.. I believe what she first says to Rhue when you catch up to her in the dream world is very telling.
"Is it... 'you' " - She recognises Rhue's (mimic's) face
"Of course you aren't him.. it doesn't matter what your face looks like." - She recognises that Rhue is not the same aura she remembers, ie Rhue knew her long before he became Rhue..
"Just kill me... I'm not afraid anymore. I've been afraid for too long." - dominant aura in Mimic at that time that she was aquinted with him possibly intended to kill her.. All other people in DE do not recollect their last moments, certainly they don't recall Rhue trying to kill them (if he was the cause of their deaths). It is my belief that she had been chased for a significant length of time that went beyond her last few moments (also backed up by her saying that she had been afraid for too long). We have never seen the Phantom Slasher chase down one individual continually with the intent to kill them over a long period of time (he normally gets them in the first go), so I believe that the dominant aura was in complete control while intending to kill her.
"You are strange... Always changing.." - For her to be saying this she must have witnessed another drastic change in Rhue's character. This change could have been natural (the dominant aura changing their view on life for example). But there is that possibility that she might have been around during a change in dominant aura.
I can't see where that last point would go for the moment, but I just thought I'd mention it.