I don't think Sauron would release Morgoth because if Sauron wanted world dominion (which he did) he would not be able to get it with the more powerful Morgoth standing in his way. Sauron would be reduced to "most powerful servant" again rather than "the Dark Lord." I think that Sauron would prefer the latter.
Here's an interesting thought.
Quote:
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For with the consent of Eru they [the Valar] sent members of their own high order...
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Would Eru have
let the Valar attack Sauron? It seems from this quote from UT "The Istari" that the Valar needed Eru's "permission" to send the Istari to M-e. So even if the Valar might have gone to M-e to go against Sauron (I don't think they would), I don't think Eru would "allow" them to. The Valar wanted the peoples of M-e to learn to cope on their own, which is why they sent the Istari.