So sorry this took so long, I bashed most of this out in like a day or two and then couldn’t get motivated to finish it off ^^;
In regards to the RP example, what I normally like to do, and this probably sounds a bit weird, but I pick a random post of theirs out and see if I can write a response to it. I’ve found that’s the best way to figure out someone’s style, I mean, sort of like you said, someone could have very long, detailed posts but in the end, it doesn’t matter if they don’t write it in such a way that it gives others something to go off. Honestly, though, I think the CS is ‘intimidating’ enough that we’ll probably only get like-minded people and not have to worry about this too much.
So with how Bella ties into it all, I have a very basic skeleton of a plot, it’s sort of similar to what you laid out. I don’t know how familiar you are with noir, but generally, there’ll be an A plot and a B plot. The B plot is the ‘main’ plot while the A plot is the distraction - basically, the story will follow the A plot until it starts getting/is solved then seamlessly switch over to the B plot.
Anyway, here’s my plot skeleton;
- Bella was raised in a catholic orphanage and was familiar with the local mob from a young age. When she was older, in exchange for jumpstarting her career, they used her as a blue picture model and a sort of ‘bargaining/blackmail’ chip, if you catch my drift. Maybe the PCs find a torn blue picture?
- I haven’t put much thought into how Bella was murdered, but potentially something violent? Obviously, nothing that would break Eve’s rules, just that whoever did it really hated her or like it was a very emotional thing.
- The PCs come to discover that Bella was engaged in a bidding war over an empty plot of land with a predominant Rockefeller-like family that far exceeds what that plot is reasonably worth. The PCs do some digging and find out that the plot was originally part of the Rockerfeller-like family’s land until it was subdivided and sold off around 1910. Since then it’s changed hands numerous times, we could tie the Detective NPC into it by having it owned by a former police chief at some point or maybe it was even owned by the studio too? Maybe some of the older PCs even filmed a movie there or something years ago. Point is after it’s sold ownership of the plot gets real shady and changes like the wind.
- Curious, the PCs gain access to the Rockefeller-like family’s house, maybe they were having a party or something - or the PCs could just simply break in. Anyway, they snoop around the house and find out that the Rockefeller-like family has a younger daughter who’s never talked about anywhere and has been missing/ran away/on an extended Europe trip since a year or two before the plot was sold.
- From here I’m not too sure how things play out, but while at the house the PCs find something that tangentially connects the missing daughter to Bella. They run around a bit trying to find out about her and how she relates to what happened, learning about the Rockerfeller-like family, etc. Eventually, they end up back at the plot where they dig up a body, or what remains of one since it’s clearly been there a long time. Somewhere on or around the body, they find a ring or bracelet or whatever with the initials of the missing daughter engraved.
Basically, the missing daughter is Bella’s mother and Bella’s father is also her grandfather or something because in these kinds of stories rich people always have secret incest babies. The missing daughter could’ve been murdered because maybe she ran away but eventually ran out of money so threatened to expose the family or something. The mob could’ve had strong ties with the Rockefeller-like family and that’s how Bella came to discover her parentage and she was murdered because she was going to do something similar.
The A plot is the murder of Bella, and the incest-baby-murder-cover-up thing would make a good B plot, I just feel like it’s missing something though. Not to sound too edgy, but it’s not hopeless/depressing/desolate enough - like it needs to be connected to something way bigger that the PCs only get a taste of but are still hopeless against. A cult of powerful elites could work I guess, Wicca was invented around the ‘30s, but that seems kinda meh and anticlimactic in a way? I don’t know, I’m just not feeling it.
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The Suspects
I read somewhere that when you’re planning a mystery you should figure out who did it then work back from there, so here’s a quick list of potential suspects I came up with. They should still work even if we don’t go with my plot skeleton or change it up until it's unrecognizable.
The Mob; Maybe they were blackmailing her and she was trying to get out or as you said, they wanted her to do something, maybe not murder but something to do with the blue pictures/blackmail, and she refused. They could also be working together with one of the other suspects, like trying to cover up for the john that killed her, or if Bella found out her relation to the Rockefeller-like family through them they could’ve been told to clean up their mess, that sort of thing.
One Of The Blackmail Victims; Maybe the mob overplayed their hand with a John and he took Bella out in a sort of ‘it made sense at the time’ type thing. Because, you know, thinking about it, even if you kill the girl they still have the photos of you two doing it, with the bonus of there now being an active murder investigation.
I know it’s cliche, but maybe she was pregnant? Or at least wanted someone to think she was pregnant? Like maybe Bella wasn’t just some innocent girl who got entangled in some shady stuff, but in actuality was running her own cons. This could also double as a motive for the mob too - they found out she was making moves on her own and put a stop to it.
An Obsessed Fan; Piggybacking off your weird fanmail idea - a creepy, mentally unstable, beyond obsessed stalker who imagined this entire parasocial relationship with Bella, like in her latest movie he believes she was professing her love to him or something.
Maybe there were rumors that Bella was seeing someone or her blue pictures became more well know among those circles and he decided if he couldn’t have her, no one could. Or something similar to the above but it could’ve been an accident? He was just trying to shoot his shot and she freaked out or he was more malicious and tried to kidnap her so they could run away together or something and she fought back. Maybe he has some obscure Bella memorabilia that turns out to be a clue?
The Owner/Financier Of The Studio; Honestly, I think this one might work better as an accomplice/someone helping out the mob or the bigshot that did it rather than the actual murderer. I think he might be a fun NPC to have around but I don't know if that’s a bit double-dipping on the Detective NPC. Maybe he might be better used to get the ball rolling - you know, maybe he smuggles the PCs out of LA and gives them their first clue/s or something. Maybe we could combine him with the obsessed fan?
The Rockefeller-like family; Literally any of the above could work with these guys. Maybe a combination of the two? Like the mob was trying to blackmail the patriarch via one of the sons or something who was obsessed with Bella. The mob or Bella got cocky and the son tried to fix the problem without running to daddy.
A Random Who Saw An Opportunity/The Detective/Reporter NPC; The premise for these two is the same and, honestly, not entirely a fan. It just feels like we’ve got a lot already going on, or at least planned, with these NPCs, and an employee of the Rockefeller-like family or something selling her out I don’t see as being interesting enough to stand on its own. It just feels better suited as like a small side plot or something.
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The Clues
I think having a visual aid of like a corkboard or something could be a really fun way for people to keep track of the clues, so it would be cool if you could do that. Speaking of clues, I’m sorry I don’t really have any of my own to add, I’m not too good at this part of mysteries :(
Letters From Bella; I’ve been flipping back and forth on this one for a while. On one hand, I like that it puts Bella ‘in control’ so to speak from the outset, but on the other, I can’t really think of a way to make it work that doesn’t feel cheaply contrived if that makes sense? Like Bella knowing she wasn’t going to make it out and setting up a sort of dead man’s switch is an interesting idea but I don’t think it works here.
I think maybe to start off we have them find/get given/whatever a single, maybe more general letter -maybe Bella was resigning for the studio or something - and then from there go into the more personal clues. I like the idea of each one having a snippet of a clue that the PCs need to put together to figure out.
Maybe as a sort of compromise, during their search, they find short notes/other letters written by her, some that highlight her relationship with the PCs? Like we could connect it to the diary and have them find a page where she goes into how thankful she was one of the PCs was there for her during a rough time, or how much she hates that one bitch.
Hidden Tape; I like this one a lot, but maybe instead of movie film they find a roll of undeveloped picture film? Just I think it might be easier for people to understand and gives us more options to work with.
Fanmail/Coded Messages; I think I gave most of my thoughts on this earlier but otherwise seems like an opportunity for a bit of fun, potentially at the PC's expense - especially with the double clues.
If we do a cult the cryptic symbols could be related to them, otherwise, they could be something as simple as noting meeting places, etc? Having riddles sounds fun, creepy or not, maybe instead of an obsessed fan, we could have an obsessed hater/troll? Like someone who wanted to freak Bella out, maybe they could have inadvertently caused her to do something that got her killed? We could throw in some poems too, maybe hide a confession in one or something? Probably not as a clue but more of a ‘makes sense in hindsight’ thing.
Diary; I think this might make for a good starting/early clue, though having them find an entire diary seems a bit brick-ish. Maybe instead they find torn-out pages she trashed or something more general like an appointment book. Perhaps they find more diary pages as they search and, like the coded messages, most seem useless at first until the end? Or they could just never amount to anything more and we just use them as fluff/building Bella’s character.
Art Piece; I don’t have anything to add to this one other than its genius and I love it. Maybe there are things in the art piece, say a house or something, that aren't there in reality and they use that to find where the bodies are buried, perhaps literally.
Summer 
Night Weather: 
Fog Moon: 
