Lockswell was the one working with Fauxnel.Rosea and Lies A LOT oh, Lieselotte really WERE innocent. Yea, durr, but IIRC, Fauxnel said that he was only partially responsible for Sir Cennair's death. One person on Gamefaqs actually had the right idea and claimed that it was an assassin. THAT is where I got this theory from! LOCKSWELL WAS THE LEADER OF THE ASSASSIN'S GUILD. THE ROYAL ASSASSIN'S GUILD!!!! He is also a member of the same fallen house Fauxnel was. Wouldn't it be in Lockswell's best interest to restore the lost House and, in a way, avenge his wife and family? To give Cheripha a Noble House to live in? The lives of Rosea and Lieselotte meant nothing to Lockswell, so getting rid of those two would mean that a member of his House becomes Archmagus and BOOM! House slowly gets restored. At least Fauxnel felt guilt for his involvement. Think Lockswell did? No. Plus, who would DARE accuse him? Who would go against the Assassin's Guild? It would also explain why no one would be believe the two girls. To do so would mean the end of their lives. Lockswell didn't get the chance to get rid of Rosea and Lisel because Cheripha deserted soon after. Plus, there's always Fauxnel to 'take over'.
If one of the girls was guilty, it would be Lieselotte.Notice how she tends to blurt out certain things that sound innocuous at first, but then when you think about it, it's a form of guilt admission or an obvious attempt at provoking someone. She said she was a "failed assassin". Well, that's certainly true in the C Path, but it's also true in the B Path. She committed the same acts in BOTH paths. It's just that in the B path, no one would have considered her until she mentioned "failed assassin" and then Tavern Lore helps put two and two together. See, she mentioned "cantrip". How did she know that? No way the investigators mentioned that detail. Maybe she figured it out on her own? Or maybe she IS guilty, but is betrayed by Fauxnel. In the Japanese guidebook, Liesel and Fauxnel's endings are said to be the only two mentioned and that they "worked together to restore peace to Artolia". No one else's is mentioned. Uh-huh. Too bad it was all in vain seeing as the B ending proved every bit as downer as the C one, except the latter one could be seen as more karmic. Anyway, that tidbit raised a lot of red flags with me, especially since Liesel is the sort would appreciate conniving "ends justifying the means" tactics(something Rosea wouldn't do even if she absolutely had to emply such tactics herself), so she would be a little more forgiving of Fauxnel's actions. Er, maybe. I honestly think Lockswell's involvement is way more likely though.
Rosea is the mortal manifestation of Hrist Valkyrie. Or Soul Jar.Notice that Silmeria has Alicia, Lenneth has Platina. Who does Hrist have? Well, there is that one woman in VP: Silmeria, but that might have simply been a disguise. In the end of COTP, Silmeria is out of commission and Lenneth is also set to rest. That leaves Hrist. So, Rosea's death is necessary for her to become Hrist. If you take off Rosea's hat and Hrist's helmet, they look frightningly similar. Even the hairstyles match. The personalities are obviously different, but that's true with the other sisters and their mortal forms. However, both Rosea and Hrist have a severe appearance, both usually surpress emotions, and they both are likely to blindly obey who they are most loyal to. Hrist is CERTAINLY like this as she does everything Odin says without question. It's not like Hrist is evil, she's just the most obeidiant of the sisters. Also, because her memories of being Rosea are likely surpressed/erased, it's expected that she'd be the harshest, coldest, and least compassionate/empathic of the sisters. She also would have chopped Lieselotte and Fauxnel to pieces just on principle. Would make an interesting VP sequel, that's for sure.