Alternate "Hiding the Ring" strategies

[Tuor of Gondolin]

But doesn't it rather defeat defeat the point of their (and particularly Frodo's) struggle and make it all rather unnecessary if Eru was always going to "save" them in the end anyway?

Actually, I factor that in. The view is what some perceive as Tolkien's
pre-Christian Christian philosophy, and it involves free will. Allowing
free will necessitates bad/evil things happening/being enacted by
"bad guys". To allow this while still not permitting it to permanently
pervert God's/Eru's design/music means that poor decisions (Feanor)
evil acts (Morgoth) do have effects, sometimes long-lasting,
but that eventually actions will be taken to redress the
balance, but there will still be great damage done. Think of Eru's
comments when Morgoth tried to take over the music of the Ainur.
And I can't find the quote now, but someone says (Gandalf?)
something like "It cannot be as though evil never was."
Also recall the longterm, permanent damage done to Ea by the
struggle between Morgoth and the Valar which left it permanently
marred, yet still essentially intact.