CommonEldarin would be the language spoken by the Elves that followed Oromë and embarked on the March from Cuiviénen to the Sea, or rather the language they developed during the March.
CommonEldarin was not an entirely uniform structure; already on the March, there were dialects.
The CommonEldarin word rondô "vaulted roof" does not contain the ending -dô; this is the stem RONO (not in Etym) with medial fortification n > nd (VT39:9, cf.