I actually think there is a good deal supporting this hypothesis.
The Mystery of the Bleeps
To begin with, Evolina and kwando noted that Raven's dialogue bleeps are actually female. Which would seem to tell us that it's a female character. Now, this could be part of a trick by Blackrune in order to get people to think that Vera is Raven. I can't deny that possibility. However, I think it seems like a rather far-fetched one, so let's assume that for now, it's at least somewhat reliable circumstantial data. Let's look at our candidates.
It could be Vera, but the Vera-Raven hypothesis makes no sense and is cheap. So let's dismiss that one. It could be Luna, but the Raven monologues don't seem to match her at all. So we can dismiss her as well. That leaves us with Valerie Princess and Candia Hind. Hind is a possibility, but it's worth noting that she's Garth's guest. Raven would want to be at the party by necessity, and thus unless Garth is an accomplice, there would be no guarantee that she could get to the party at all (Admittedly, Hind did "follow Garth"- likely meaning she requested to come- but she still couldn't have come without Garth's permission). Invoking additional accomplices when you don't need to artificially increases complications and thus should be dismissed as a matter of course. In addition, since Garth seems to have really wanted to say something to Vera about JP (at least, he heavily implied that during their initial conversation), it seems rather odd to argue that he is a member. So that seems to leave us with Valerie.
The more I think about it, the more Valerie's name strikes me as a bit odd. Why would you go out of your way to emphasize that Valerie's name is the same name as that of a famous chessmaster? There's no other inherent joke there- at least not one that I can see. So... it feels like that name must have a reason behind it. Evolina has postulated that Valerie IS Valerie Princess after all, but that makes little sense to me. After all, she doesn't seem to have much reason to hide that- Ost went out of his way to talk about how he wanted to invite famous people, after all. And everybody in the room is knowingly famous, so it's not like she would really stick out.
So I can't help but wonder... is Valerie Princess actually her name? Or could it be Raven's pet pseudonym? To me, that seems to make a good deal of sense: what name would suit Raven better? It also seems to make Tief's rather odd "Oh, everybody knows who RAVEN is, it's so obvious" comment make a bit more sense. Sure, he could have just said that entirely to mess with Vera, but I can't help but wonder if there wasn't a grain of truth in there (particularly since he felt the need to differentiate Raven from Vampire; if you're just messing with Vera... why single Raven out?)- that maybe he was subtly teasing her: "It so obvious, even her name is a clue!"
And then you have Joseph's line at the end of the investigation:
Joseph (?): You think I’m out of cards?
Maybe you should just stick to chess.
Joseph (?): From my experiences chess players are quite appropiate company for you.
I still feel like this comment has to be a clue. It's random; particularly that last line. That last line about company is wholly unnecessary if Joseph was just talking about Raven's intellect/the way she plans her crimes. It feels like it supposed to be teasing Raven about something, and really the only way that comment makes any sense is if the person has some kind of connection with a chess player- and the only one we know about is Valerie. I'm not sure how some parts of the line fit in exactly (which does make me slightly skeptical), but let's cross that bridge when we get to it.
The Tarot Reading
I want to take a look at Valerie's reading again-
Princess: The Lovers. A card of change. A card of relationships. It symbolizes being driven by desire or curiosity. Desire for pleasure. This desire may give her the impulse to do something. What is this impulse? There may be temptation waiting in the future. Doubt, and a choice she has to make. A dilemma is waiting. A dilemma born from relationships. How will she choose?
Now first I want to note that if we view Valerie in isolation, this card seems to make little sense. The only person who Valerie seems to have a relationship with at this party are Ost and Hu, both of whom she appears to dislike. Whereas much of the game seems to center around Raven's relationships- their likely relationship with Tief (who is incredibly likely to be Raven's butler), and their relationship with both Lira and Vera- the two who are trying to find Raven- partially motivated by Raven's murder of both their parents. I will admit that I'm not sure how Raven would really be connected with change per se.
There does seem to be a way out here though- Valerie DOES seem to be very concerned about romantic relationships, about being alone and single, etc. So the reading could easily also refer to romantic relationships, and this is far from definitive.
Now, desire for pleasure really fits Valerie's personality regardless of whether she's Raven or not. However, this desire gives her the impulse to do... something. Something that gives her pleasure. Now, this could certainly be a reference to her gambling, but I can't help but wonder if it isn't her impulse to commit murder... and do so in a seemingly perfect way. Adding an additional gamble- that of being able to get away with a seemingly perfect murder- seems to fit Valerie's personality.
Next, there's some kind of choice she has to make due to a dilemma. Now, again, we have the issue of the fact that Valerie doesn't seem to be in a dilemma. Yet, we know Raven is. Raven's perfect burial of the mystery genre is apparently being tarnished, and she's having to work around that. "Cheat around the board", as Joseph put it. The choice she has to make is still unclear, however, and so is how doubt would feature into Raven's scheme: she certainly still seems to want to go through with her plan. These invite some skepticism. And how is Raven's dilemma born from relationships? (I actually have an idea about this one, but I'll touch on that in the second half of my post.)
So while there are multiple interpretations and explanations for some of the reading (and the rest could refer to data we don't know about yet), it still does seem to be quite interesting... and a good deal of it does seem to fit Raven's personality/situation.
A couple other curious things
Now, neither of these is definitive, but they both seem rather curious to me. First, you have one of Raven's crimes: it takes place on the building of Hu's company. A company Valerie just happens to work in. (Admittedly, this is actually a big problem of mine: an office worker seems like a weird cover for Raven, but maybe there's more to it than that.) It strikes me as an odd coincidence.
Also, we still don't know why Ost invited Valerie. Valerie says that they met at a casino- but refuses to explain anything further, and now seems to view Ost with disdain. Yet... Ost still invited her (for reasons he refuses to divulge), and Valerie still agreed to come. It feels like there has to be some connection between these two that we just don't know about yet.
Also, there is one aspect I thought about in our Xat conversation that kwando didn't touch on:
Let's look at some of Raven's earlier monologues again. We'll start at the beginning-
???: It is a good thing I am not trying to write a detective story.
???: If at all, one could consider this their burial. Heh, an interesting way to put it...
???: Rest in peace.
So... what Raven intends to do is set up something that isn't a detective story; a story that doesn't abide by mystery rules and, in fact, "buries" the detective subgenre. Exactly what that means yet I can't guess, but that's not what I want to talk about. Let's move on.
Monologue #2 is when Vera falls asleep on the train after meeting with Lira. And this is where things get interesting:
???: No good...
???: This story is no good...
???: There is no hope...
???: for the detectives...
???: Its too bad...
???: But its too late...
So... even after Vera and Lira agree to go together... something appears to have gone wrong with Raven's plan. This story is suddenly "no good". And there is "no hope" for "the detectives" (exactly what this part means is also unclear.) Let's move on.
Again, Monologue #3 operates under a lot of vagueries and unknown contexts, so I can't make sense of most of it. But, we get the same theme:
???: Why? Why do you have to corrupt my mystery?
So, something or somebody has corrupted Raven's mystery. Has ruined it. So we now have to figure out who, and then maybe we can deduce how. I think timing is important here. Raven was saying that the mystery was corrupted the day before the party itself. So... whatever factor it is has to be something introduced prior. Which, to me, seems to leave two options: one of the other guests is bringing somebody who will wreck Raven's plans... or Vera and Lira coming together has done massive harm. I can't help but think it's the latter, due to the timing and the way the whole thing is framed.
Similarly, let's take a look at the dream sequence at the end, where you get a similar trend.
Joseph (?): It’s already stained. The rules have been violated.
Joseph (?): Why should I adhere to rules you have already broken?
Joseph (?): ...I get it. We both have no clue what went wrong. Perfect.
The mystery has been "stained". Rules has been "broken". For some reason that both Raven and Joseph are unaware of/couldn't stop/couldn't predict. The first thing that comes to mind, again, is Vera and Lira teaming up.
This is where things get a bit weird though, and make me question the above hypothesis- if it was Vera and Lira teaming up... Raven seemed to know about that. Why are they acting in confusion about it now? Is it something else? Or... did two things go wrong, one thing Raven knows about and one which they don't? Hm. This is a point that I'm unclear about; as I am unclear as to how Vera and Lira teaming up seals their fate- hence "there is no hope for the detectives". However, it does seem like a decent lead to pursue.
I also heavily suspect all this mythological quotes mean something- as do the Judgment Seekers, but that's a discussion for another time.