If the target creature is an illegal target by the time Echoing Courage tries to resolve, the spell doesn't resolve. No creatures get +2/+2.

A face-down creature has no name, so it can't share a name with anything. This includes other creatures with no name.

Echoing Courage has only one target. Other creatures with that name are not targeted. For example, a creature with protection from green will still get +2/+2 if it has the same name as the target creature.

Unless a token is a copy of another permanent or was explicitly given a name by the effect that created it, its name is the subtypes it was given when it was created plus the word "Token." For example, if an effect creates a 1/1 Soldier creature token, that token is named "Soldier Token."