>is Adenydd a legitimate Welsh name that has been used (and can be used)?
No. As Claudia said, 'adenydd' = 'wings', it's never been used in Wales as a name as far as I know, and it doesn't show up in any records. There are quite a few of these 'Welsh names' lurking in baby-name databases that are never used as names in Wales, including several in here! - so it's always wise to check.
◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄◄
http://www.behindthename.com/pnl/65849