I have been researching Ann Brett and think I can help your query. The Ann that William Green married in Shipdham in 1831 was from Hindolveston and was born earlier c.1807 (see later censuses for Cranworth where they lived and raised a family). She may well therefore have been Blunkall by birth although I cant evidence this.
Ann Brett from Cranworth/Shipdham married a different William Green in Tunstead district (Happisburgh?) in 1838 (source free BMD). He had been born in Devon. The rest of your post is correct.