I can feel your pain, a customer just wanted to get rid of his old Outlook e-mail because it was "overtaken by potatos" as he said. BUT: That address is the Microsoft account he uses for signing in to Windows and Edge automatically signs in with that...
To start with, why on Earth do you allow Mother to use your account? Does she open your mail as well? Creating another account for her with limited privileges would both allow for her to get all the Edge garbage she wants and still prevent her installing anything cross-account apps or programs including browser add-ons.