Many seem to think that the creation of Satsuki's route is what's making things take a while, or perhaps even Kohaku's, since her's was rushed way back. But I have to wonder if it might have to do with Hisui's route instead.

It doesn't seem like it fits in well with the direction TsukiRe went. Shiki being trapped in bed for half or more of the route, wasn't necessarily so bad in an 8 (or less) hour route, but if it's around 20 hours, then it may be egregious. There's also the fact that besides the original Ciel, Hisui was the least popular of the 5 main heroines. Even the spinoff content seemed to have trouble doing much with her, whereas Kohaku had a lot of action. I wouldn't be surprised if they totally revamped her route, especially since the ending of her original route may not hit as hard as it once did. You could argue that the other routes will be revamped, but even the skeleton might be replaced, in Hisui's case.