"How does one pronounce Tolkien?"

[Mnemosyne]

Depends. The difficulty is that the pronunciation guides were published as part of Lord of the Rings, albeit in the Appendices. Which means that they are not placed nearly as high on the hierarchy of accuracy as, say, the doors to Moria or the maps or the fact that Aragorn had grey eyes.

But how many visual cues about Middle-earth came from Tolkien during his lifetime?

I think the difference is that envisioning something from a book takes much more imagination--and thus much more effort and wiggle room--for the reader than simply pronouncing a foreign word. You're given more liberty simply because of the differences in medium (so, we could have two completely different-looking, but equally canonical "Frodo"s but however we say his name it's going to sound pretty similar).

The other thing is that we're given guidelines--which Tolkien says we don't have to follow if we don't want to!--in the books themselves, and we know that Tolkien expended a lot more effort into languages than into illustrating (though he spent a lot on both!).

What would be really interesting now would be to hop 20 years or so down the road and see how much the "your mileage may vary" attitude towards mental pictures remains as more and more people enter the fandom with the Jackson films ingrained in their heads.