The information about
Eider as feminine of
Eder is not correct. As you can check at Euskaltzaindia (
http://www.euskaltzaindia.com/eoda/izenak.asp?onoma=izenak&hizkuntza=es&testua=eider),
Eider is a newly made name in Basque, not from
Eder but from the European bird name and/or from the Gaelic
Eideard.
And about the rules for feminizing male names, some names just change the masculine -a to an -e (
Koldobika > Koldobike), other names add an -a (Sabin >
Sabina)...
Lumia
http://onomastica.mailcatala.com