The supine is never nominative - only accusative and ablative. Correctly, it's the perfect passive participle.
Not quite. If it were mereo, mereri, meritus, then it'd be semi-deponent. However, I see two different entries for mereo - mereo, merere and mereor, mereri, most likely having both the deponent forms and regular forms. Also, meritus seems to be used both active, passive and middle tenses, which is quite a large difference. Did the wings earn, or did someone earn the wings? Chambers-Murray cites the middle tense for meritus as most frequent.
Personally, I'd stick with perfect active: Pennas Merui, although Pennis Meritis sounds better, if you can get past the ambiguity.
One wing only?
Who else?
How odd - why did my quotes not work properly?
[edit - PP... one too many /quote ]