I am pretty sure that woman was called something else.
Myfanwy doesn't sounds like "may-fanny". Thank god! I love Eilonwy and the W should be pronounced as well.
"May-fanny" is beyond awful it makes me think of a fanny, and well,
May. It is really odd.
Myfanwy should be mih-VAN-wee. A link is here:
https://www.britishbabynames.com/blog/2014/04/welsh-diminutives.html (you can listen to it, scroll down to Myfi and read the entry).
Myfanwy with the correct pronunciation is fine but not a favorite. I love Eilonwy.
MAY-fanny is terrible.
My guess is that she wasn't Welsh but a tourist and that her parents had tried to give her a Welsh name but mispronounced it? Or that she was Welsh but her parents didn't speak Welsh? Or maybe she had a very similar name?
Maybe she was actually May-Fanny? Depending on how old she was
Fanny could still have been a somewhat normal name when she was born. Or maybe you misheard it.
Similar names that I can think of:
MayMay-Fanny (oh no)
Maryvonne (but this sounds really different)
MalvinaMaeve - this is sort of likely, maybe Maevie was her nickname or something?
Mayfair
Mavis
Please rate my list:
https://www.behindthename.com/pnl/6232