The fan-fin forum is a relatively new creation and still pretty limited. I don't think you'd be able to able to recruit an author for your fanfic there.
However, as to the second part of your question - the Downs is definitely a good forum for having people fact-check your outline and point out canonical sources. There are plenty of very smart Tolkien erudites and scholars here, you would get some good feedback on the canon aspect.
But, saying that - fanfics aren't expected to always be perfectly 100% canonical, that's not their purpose. They build on the canon, and some might even deviate from canon, and that's okay. That's why they are fan-fics.