***SPOILERS ***
A year before the events in which the film took place,
Baron Otto murdered Irena's father when he would not consent in giving her hand in marriage
and placed the blame on the local vampires. The Professor and Inspector hired actors to play the role of vampires in order to frighten him and to solve the case.
Also, we don't see Mora or Luna transform from bat to human being or the other way around. These scenes are visualizations of what people claimed to have seen.
The scene where we see Mora enter the house as a bat, is an event described by the two servants. These two servants are part of the plot, so it is exactly what they have been told to describe.
As for Luna 'flying' down, landing next to Mora: at the end of the movie we see her packing these wings. This event was put in scene to delude the murderer.

Or when Mora and Luna walk through the spiderweb without disturbing it, as Dracula did in Universal's picture (though to be fair, it later on appears as though the Baron and Inspector do the same). Despite it being a remake, it gives the impression that it's an ending that was tacked on as an afterthought, even though it wasn't due to 'London After Midnight'. Poorly thought out in that regard, I agree.