-s-jo-/-s-ja stems do produce -dd- in Welsh (*Sasia behind
Welsh Saidd, Corios behind Cordd) - but there might also be cases where
-dd- does not develop - for example PIE *H1eseH2s "her" perhaps becomes Gaulish
Eia and Welsh Ei "her" due to later Celtic loss of internal -s- (ex: Common
Celtic *Suesor "sister" becomes Late Gaulish Suior, Welsh Chwaer,
Irish Siur/Fiur) - unless, of course, Eia is modelled on a form
without internal -s- which is also possible.
K.H. Jackson has Welsh DD (earlier D) from -j- occuring in the
fourth or fifth centuries AD.
Gwynedd is found as Uenedotis in inscriptions from the Fifth
Century AD. This must come from an earlier Common Celtic *Uenedatis /
*Uenesiatis / *Ueneiiatis (-atis is a locative stem in Celtic) - in any case,
all three forms are likely from PIE *Wen-, but are not direct developments from
*Wen-et-.