The character designs here are great. Marisa looks so badass in that cloak-like dress along with her comically large hat. Fusing Gensokyo with some late 19th/early 20th century technology makes it look something straight out of the Meiji/Taisho era (which I assume is the point). Besides the aesthetics, one gripe I have with these sort of whodunnits', is that you would never be able to figure out within reason what could be the causes, since the reasons created are so obscure and contrived. Still a fun doujin.
Also btw, dunno if anyone will see, but the lower left panel on page 3 seems to be a duplicate text bubble, unless Reimu is repeating back what Marisa said to her (which I would assume would have something like "huh" at the end of the phrase, to question what Marisa said).