Like I said in my other comment, I found it, and I have more feedback. After reading every Tolkien canon scrap I could find about rune-Rumil, I honestly don't think your interpretation of rune-Rumil is that AU. It doesn't seem to me to contradict the little canon there is about Rumil, beyond his being unable to talk and hear, which would make talking to birds and beasts irrelevant. The escape from Morgoth is still there, as is his invention of writing.

Another thing your Rumil reminds me of...and I mean this in a good way...is Frankenstein. The lank hair, the hollow cheeks, the height and broad shoulders and big hands, the isolation and horror, the parallels with monstrosity.

Now I'm off to read Tuxedo Elf's story.