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The concern about becoming another tyrant dealt with someone overthrowing Sauron "by use of the Ring". The Valar could have sailed in (as they did in the War of Wrath) and done the deed without that danger - hasd they chosen to.
Your second reason, Mithalwen, comes closer to explaining it, I think - but is still only a part of it. Christopher Tolkien comments in Unfinished Tales, in the essay on the Istari, (and, I believe more is in HoME) about the Valar rethinking their ancient decision to try and shield the Eldar by their main force and glory (through the invitation to Valinor, primarily) and their conclusion that such was, in the end, an error - not in accordance with the purpose of Eru and not for the ultimate best of the Eruhini (The Elves and Men).
We could speculate about why such might have been the case - that the Elves first and later men (as the "Dominion of Men" came and the Elves faded) should make their own ways in the world not under wings of the Valar who would smack down any threat or evil that might threaten the children.
But the essay makes it clear that this consideration was behind the prohibition against Gandalf and the others trying the master other peoples through raw displays of power and glory. So it seems pretty plain that the same prohibition would apply against Orome (or Tulkas or even Eonwe) sailing in at the head a vast army of Maia to rescue the Elves and Men from Sauron's shadow.
So, I would say your second reason is completely true - as far as it goes. Only, I think the truth includes what you said and goes a bit further.
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