Wow! This is more complicated than deriving Daisy from Marguerite ... nice try but, I think, no cee-gar. Margaret also gives you Meg as a nickname, and most nicknames come from the siblings of the nicknamed - especially if the name is long and complicated for infant mouths. All those Elizabeths who turn into Betty, Libby etc! And little kids are alas well known for getting colds, nasal congestion etc, which would give you Peg from Meg with no difficulty at all. Same, I think, with Polly from Mary, Bill from William and Bob from Robert, as I seem to be saying on this board rather frequently. But I did enjoy your reasoning!