Is the following sentence correct or incorrect - if incorrect why is it incorrect - 'What the exact mix of experience and natural talent that made Austen the brilliant she was may never be unknown'?

"What the exact mix of experience and natural talent [was] that made Austen the brilliant [writer] she was may never be [remove un]known."

As for why it is incorrect... because it is confusing, because there is nothing for brilliant to modify, and because "may never be unknown" is opposite to the contextual meaning of the sentence.

The added text in brackets fixes the grammar, but the sentence is still rather awkward. I'd reconstruct it this way:

What the exact mix of experience and natural talent was that made Austen a brilliant writer may never be known.

Or, more simply:

The exact mix of experience and natural talent that made Austen a brilliant writer may never be known.

Or even:

We may never know what combination of experience and talent made Austen great.