[Inziladun]

Originally Posted by tumhalad2

I said I'm assuming Eru has the characteristics of the mainstream view of the Christian god.

It is a logical assumption, given that Tolkien was a devout Catholic.

Originally Posted by tumhalad2

I argued that if we assume he has these characteristics, then it follows that he is subject to the same kinds of logical contradictions that the Christian god is.

First off (and I have no intention of debating the matter with you here), it is by no means a universally accepted conclusion that the Christian God exhibits "logical contradictions". That is your interpretation. You're entitled to it, but don't expect everyone to agree.

As far as Eru Ilúvatar is concerned, you've got a bee in your bonnet because you don't see Divine Providence holding Túrin's hand and pointing him in the right direction. Why? Is it the duty of the Creator to yank someone back every time they get close to the brink of disaster through their own doing? No. The One may give someone signs to guide him, but it is incumbent on the individual to recognize them, and to alter his behavior accordingly. That is free will.
I would argue also that Túrin did have experiences which should have turned him away from his path. What of the tremendous good fortune that Nellas happened to be watching when Saeros attacked him first? That "chance" happening completely cleared him of wrongdoing in Doriath. When he was told by Beleg he would be welcomed back there, he spurned the offer, saying

'I will not pass into Doriath, and make use of Thingol's leave and pardon.'

CoH

That choice eventually led him to Nargothrond. Gelmir and Arminas were sent to Orodreth by Círdan to bear the warning of Ulmo there, telling them to shut the doors and stop drawing attention to themselves. Did Túrin listen?

'What does Círdan know of our wars, who dwell nigh to the Enemy? Let the mariner look to his ships!'

CoH

Arminas explicitly rebuked Túrin, giving a very accurate judgement of him.

'But you, it seems, will take counsel with your own wisdom, or with your sword only; and you speak haughtily.'

CoH

And what happened? Nargothrond was taken by Morgoth's forces, and all Túrin's pride was in vain. So Túrin never had any guidance, then?

Originally Posted by tumhalad2

So - this "fate" is not actually benevolent?

"Fate" is only so when speaking of what the Creator already knows his Children are going to do. Having knowledge of their actions and not interfering in them is merely another allowance of their freedom to act, for good or evil. If you don't think that's fair, or right, again, you're entitled to your opinion.

Look, in LOTR Frodo and Company have Gandalf to advise them, but in the end it doesn't make a difference that he was a Maia. Those that he advised, with the possible exception of Aragorn, didn't know he had any "inside information". They used their own wisdom to make their decision to listen to him.
Also, Frodo wasn't "fated" to be the Ring-bearer: he was the chosen instrument for the task, but he still had the free will to refuse. That is made clear to him by Gandalf:

'And now', said the wizard, 'the decision lies with you.'

LOTR The Shadow of the Past

And again by Elrond at his Council:

'But it is a heavy burden. So heavy that none could lay it on another. I do not lay it on you.'

The Council of Elrond

If Frodo had refused the Ring, I feel certain "fate" would have been altered, and the One would have found another means to accomplish Sauron's defeat.

So you see, "fate" is only a constant in the context of our own actions. I don't believe Túrin was "fated" to have the life he did, just as Frodo was not inexorably locked into going to Mordor with the Ring. Choices are the genesis of "fate".