The Grand High Witch's expressed distaste for garlic is a nice acknowledgement of how this plant has typically been considered as a means of warding off evil in most cultures. It may be typically regarded as a vampire repellent, but the idea of witches being repelled by garlic has a long-standing tradition that is simply not commonly acknowledged in most modern popular culture.
The Grand High Witch keeps a bottle of Formula 86 in her metal breast plate, and when she pulls it out, it's covered in ice and steam coming off it like it just came out of a freezer. You ever hear the expression "it's colder than a witch's tit in a brass bra"?
The Grand High Witch's change into a WomanchildButt-Monkey may seem out off blue at first, as it makes her looks less threating to the audience, but if you notice certain patterns in her behaviour (acting in such a Large Ham way, throwing a tantrum whenever things don't go like she wants, crying in pain when she gets hurt), you realise that they resemble that of a Spoiled Brat, the exact thing that she loathes and hunts over. Which makes it both ironic and brilliant!
Fridge Horror
Grandmamma tries to order the split-pea soup the witches are eating, but is turned down because it is special-made for the group (they requested it without garlic, and the waiter insists that it's very bland as a result). Grandmamma knows full well what was done to the soup, but asked for it anyway. Did she want to join her grandson as a mouse?
She may have been making sure that it wasn't available to order by just anyone. Had she been allowed to order it, then she would also have to find a way to make sure that no one else could either outside of the witches.
