The movie was filmed in Oklahoma
Agatha's speaking cat's name was Eurydice B. Magillicuddy, it is not clear how that become Mrs. Mac. Who gave voice for the cat has not been credited.
Roman Numerals: MDCCCLXV = M (1000) + D (500) + C (100) + C (100) + C (100) + L (50) + X (10) + V (5) = Year 1865.
At 1:03:34, Agatha says to the alligator "Thanks, Marielle', so that is what they had named the alligator.