At first, there being a witch in the hotel kitchen preparing the witches' food seems like a Contrived Coincidence (just so someone 'good' doesn't accidentally get turned into a mouse) but makes a lot of sense. Of course the witches would want one of their own to prepare their food since there are known witch hunters out there. A witch in a hotel kitchen is also in a very good position for getting to children.
The witch in the kitchen may have been the reason that specific hotel was selected for the meeting.
At the end of the film, why doesn't the Grand High Witch use her eye beams on her arch-enemy Grandma before getting turned into a mouse? At that point the masquerade was broken and she was being in mortal danger... unless it is a power that can't be used more than once in a day.
Considering the intense focus in her manner, it may have been that the Grand High Witch was focusing her powers on keeping herself in human shape and couldn't spare the energy for that, particularly when it took something as simple as a bite from Bruno to make her lose focus.
