Father Brown tells Mrs. McCarthy that the name Abigail is Hebrew for beautiful. The name actually means "The Father Rejoices".
The used poison, Potassium ferrocyanide, isn't poisonous. In particular not in the quantities shown. It gets excreted from the body via the kidneys completely unchanged as it is a very stable molecule. In fact it is used even today as anti-caking agent in table salt as E536 in the EU. It gets sometimes even fed to livestock.