I just replied on the other board, but I repeat here the argumentation about Avril and Averil(l).
Avril, Averil, Averill (and Averell) and Everild are all legit forms from the Anglo-Saxon name Eoforhild. The serie could be, more or less, Eoforhild>Everhild>Everild>*Everil>Averil(l)>Avril .
Avril, reflecting a contracted pronunciation, coincides formly with the French avril, "April", but it is just a coincidence. It is true that most of the parents will select Avril because "hey, it is French and that is cool", but this has nothing to do with the legitimacy of the form (only with parents' knowledge, trendyness, taste...). On the other hand, historically Avril was used as masculine name in French (just as Aprilis in Latin and Abril in Catalan, Spanish and Galician).
The surname Averill is a matronym (the mother's name used as family name, with or without a mark) and was a first name before a surname. So, it is true that it is a surname, but it is also true that it is a legit first name. (Another question is if it is a good name or is not.)