Etymologically ignorant people and tryndeez aren't confied to North
America (though they do
seem a lot more common here!), and I think Welsh folks who use
-wyn for girls either aren't aware of the grammatical/etymological inappropriateness of it, or don't care. In any case,
-wyn for girls is a 20th-century phenomenon, and to me is like using
Francis for a girl in English *coughlikeBrookeShieldscough*.
See here for a more detailed discussion on
Branwen,
Bronwen, and other like names:
http://www.medievalscotland.org/problem/names/branwen.shtml. They also briefly discuss the
-wyn problem in their article about
Ceridwen and related names:
http://www.medievalscotland.org/problem/names/ceridwen.shtml.