Dear Patrick,
The RV form a-vadya-gohana more likely means something like "hiding the unspeakable," that is something publicly embarrassing or disgraceful, rather than hiding a generic flaw per se. cf. the decompositional form in RV 4.18.5:
avadyám 'va° mányamānā gúhākar | índram mātā́ vīríyeṇā nír̥ṣṭam |
áthód asthāt svayám átkaṃ vásāna | ā́ ródasī apr̥ṇāj jā́yamānaḥ ||
Like (when) one believes something to be unspeakable, the mother made hidden Indra, (thought he be) flowing with manliness. Even so, he rose up, donned his own garb, (when) born he filled up the two howlers (= world-halves).
Regarding atisiddhi, I wonder if it's not an extension of the triplet we find in Pāśupatasūtras of atidāna, atiyajana, atitapas about which Minoru Hara has written (re-published in Pāśupata Studies edited by Jun Takashima). That is not perfection of X, so much as extra-X as polemical against those who merely do dāna, yajana, and tapas.
Best,
Caley