The first problem is to caracterize Varuna as a "god",
with all the anthropomorphic content this means.
This word "varuna" may just be a concept and Mitra "friend, contract", with
which it makes a pair, is more a concept than a "god".
What is conspicuous is that Varuna is the "god" of rta,
and Mitra is coupled with Asâ in Iranian.
Only the word "varuna" may be purely Indic,
but the pair mitra-varuna is much older than Indo-Iranian.
Dumezil traces this pair up to proto-Indo-European thought.
The Germanic equivalent is Tyr-Odinn.
Arnaud
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As always, this is mere speculation. I have quoted you the references from RV, which recognize Varuna not just as God but as king of Gods (and of men, of course).
Kishore patnaik