> Rumia has relatively short hair
> Flandre has long hair in a side-ponytail
> Reimu and Marisa first meet Rumia and then Flandre in the time-span of a few hours.
As you can imagine, I'm kind of skeptical about this theory, unless vampire hair grows like weed on crack.
Actually, it has to take longer than mere hours. It's night in the first stage, daytime in the second and at least sunset in the third, if not a future day/night transition. On average, at least six hours seems to be lapsing between bosses. Since Imperishable Night establishes the use of a normal 24 hour clock within Gensokyo's borders, I'd be surprised if the girls completed stage 4 in less than a day's time and once you reach that point, it's hard to tell because:
A. You're indoors, so you can't estimate the time lapse via lighting conditions
B. Space in the library is warped so how much further you have to traverse is indeterminate
C. You meet Sakuya immediately after that and since she can warp time, it's difficult to tell exactly how long the fight takes. It could be a very short or very long amount of time depending on exactly how her ability to manipulate time works. Granted, I'd think it to be shorter
Furthermore I think it's implied that even more time lapses between the main game and the extra stage isn't it? I don't recall if the exact amount of time is specified though but if it's not, the evidence isn't very conclusive either way. Moreover, the ribbon Rumia seems to be wearing is very suspiciously placed on the same side of her hair as Flandre's side tail. Who's to say she didn't just tie it up to conceal her identity?
Nevertheless, I remain skeptical for different reasons. Firstly if fixing the problem was as simple as putting a ribbon amulet on Flandre's hair, why would she still need to be kept under lock and key by the time you meet her? Secondly, it should be relatively well known from her exchange with Marisa that Rumia's something of a living crucifix, more or less. Vampires don't mix with crucifixes much better than they do sunlight, do they?
Just as an interesting side note though: If we were to take a chain of alter egos which lead from Rumia, to Flandre to Nue, it would seem to me that Rumia's the Anti-Saiyen, transforming into multiple forms with black colored hair being more powerful than blonde. :-P