Ok. Confirmed 2 things. Apprentice backpack was where apprentice journal was in Whispering Depths, so it seems clear Eliette was not the apprentice of the Red Wizard, like you said.
I wonder if they had the journal elsewhere originally because I could have sworn it was with the Eliette journals and hence the confusion.
But a Red Wizard is Mulan human, and Eliette was high elf, so that coupled with the apprentice backpack near a dead body seems pretty clear that the apprentice was escaping and was killed by Eliette.
As for Shadowheart, yeah, I triggered her classroom scene again. Seemed pretty clear they were suggesting the kids were kidnapped by Dark Justiciars since SH says it seemed like the work of her brothers and sisters.
Also, it triggered her strange memory magic moment, so the theory about her being one of the kids originally does hold a lot of merit. It seems almost as if Selune's power tried to course through her while she was reading the school book about the abducted kids and their families. Then she said memories were trying to resurface. Hmmm...
Her magic moment only occurs around something related to Selune, so...