I thought I'm through with explanations, but it seems I'm not...
The purpose of The Sil/COH: DEFEAT MORGOTH. Plain as that. In COH it expands to AVENGE ME AND MY FAMILY.
Turin has just as much 'right'/choice not to listen to Melian as Frodo has not to listen to Gandalf, and vise versa. The fact that the outcome of Melian's and Gandalf's advice is different doesn't make their roles totally different. Because what you said is that Melian cannot be qualified as a 'guide' (like Gandalf) because Turin didn't listen to her. Look, if a student doesn't follow a teacher's instructions, they're still student and teacher, aren't they?
It's true that Gandalf was sent with a mission, and Melian was just there. Well, it's just like being asked to work vs volunteering to work. The work remains the same.
You are saying that if a god is perfect, he would't allow suffering. But a perfect world isn't one where there is only happiness. It is a balanced world, where you can see the
differences. You don't know what happiness is until you compare it with sorrow/suffering/etc.
Turin is just one person out of thousands that suffered in ME. He
is sort of special, because his story is even more ironical and tragic than most others. But that doesn't ean that he's the only one suffering. The Narn describes the
story in great detail, but Tolkien never singled Turin out the way you make it sound.