It's pretty clear at this point that our main group of characters are all from different points in time, so I figured I'd try to hash out exactly when they're all from
(Also, I'm no history buff, nor a historical fashion expert, so if anyone has any insights to share, please feel free)
Morella is the one we can pinpoint most precisely: her flashback puts her pretty clearly in the Irish War of Independence, which was waged from January 1919 to July 1921. Narrows it right down to a 2-and-a-half year period of time.
Duke also seems pretty easy to pin down. He has a coin from 1912. It's unusual to get a coin the exact year it's minted, so he's probably from a few years after that, but likely still in the 1910s sometime.
Berenice is another easy one. Again, I'm no fashion historian, but her outfit in the party flashback, as well as the outfit her spectre form wears, reads pretty "flapper" to me. So 1920s.
For Eulalie, the outfits and hairstyles we see in her flashback look either 1940s or 1950s to me, at least based on google searches. That would fit with her saying that a coin from 1912 is pretty old. I've seen a few people in the comments of the webtoon theorising that she's a post-WW2 Japanese immigrant to the US, and that would fit with this too. If I had to pick one decade, I'd go '50s, but it could be 40s. Edit: i have been informed in the notes that the '50s is the more likely of the two decades.
Lenore and Annabel are in the 19th century for sure, but I don't know enough about historical fashion to pin down exactly when. They have railways, but not cars, which puts them between the early 1830s and the early 1900s. My best guess, based on googling the fashions of different decades, is the 1880s, but I could be wrong.
Edit: I went back to reread a flashback chapter and immediately proved myself wrong. In the flashback where Lenore, as Leo, meets Annabel's father, he's talking about the White Star Line and the ship the RMS Oceanic. The maiden voyage of the Oceanic was in September 1899. Cars started to become a widespread thing in the early 1900s (they became accessible to the middle class in 1908, but rich people would have had them sooner), so that leaves us a very narrow window indeed. 1900, 1901, maybe 1902.
Edit 2: Mr Whitlock refers to the Oceanic as the largest ocean liner in the world. This was only true until July of 1901, when the RMS Celtic was completed. We know that this conversation happened after summer, because Annabel spent the summer with Lenore. Which means that that conversation can only have happened in 1900, towards the end of the year. The wedding was in winter. So they died between December 1900 and February 1901.
Edit 3: Now that Ep 158 is out, it confirms that it's after the holiday season. So we're in 1901 without a shred of doubt. It's likely January during this conversation, given that Lenore felt she couldn't wait much longer after said holiday season. Factoring in courting time for believability, I'd imagine the wedding was likely in February.
My estimate for Prospero is more guesswork than not, on my part, but I think his clothes are right for the 1900s. According to google, that would fit with the usage of the slur in episode 112 as well. There was a plague mentioned in the flashback we saw, and there was an outbreak of bubonic plague in the US (specifically centred on San Francisco) from 1900 to 1904.
Pluto we know is from a later period than Duke, but probably not quite as late as Eulalie, because when she calls Duke's 1912 coin old, Pluto's response is basically "yeah, kind of". The 1920s or '30s are most likely, then. I'm gonna guess 1920s, but if anyone can narrow it down, please feel free to do so. Edit: That train ticket that Pluto has in the ep83 flashback has "L. M. & S. R." at the top. That's almost certainly for the London, Midland and Scottish Railway company, which was first established in 1923. it doesn't help narrow things down much, but it does at least confirm '20s or later.
Ada is wearing a bra in her episode 110 flashback, so she's from a time after they had come into common use (and after corsets had fallen out of use, which fits with her earlier comments about never having worn one), which google tells me happened after WW1. As far as I can tell via google, her maid's outfit seems to fit both the 1920s and 1930s. I'm guessing 1930s, but I could be wrong.
Will is listening to the radio in his flashback, which (again according to some google searching) would put him in 1920 or later, assuming he's in America. Honestly that's all I've got so far for him. Someone else might be able to pick out other clues, but I got nothin'. I'm gonna guess 1920s for lack of other information. Edit: thank you to the people in the notes who have informed me that Will is likely in Dustbowl Era Kansas, i.e. the 1930s.
Montresor is the most difficult, appropriately enough. That he's from the Wild West era seems fairly obvious, but that's a large swathe of time, and I wouldn't know where to look for clues to a specific decade. I'm going to guess 1890s, entirely based on cowboy media set in that time, but I could be way off.
So my current guesses, from furthest back to most recent (revised, post edits above): Monty in the 1890s, our leading ladies in February 1901, Prospero roughly between 1900 and 1904, Duke in the 1910s, Morella specifically from 1919 to 1921, Berenice and Pluto in the 1920s (albeit in different parts of the world), Ada and Will in the 1930s, Eulalie in the 1950s.