"From unattested..." is just commentary from Paarthurnax. It's not canon. In the Official Source section, you just have "unseen". It's entirely possibly that nun might be "seen", but we don't have in-game usage, so anyone who comes across "nun" wouldn't be able to immediately cross-reference what it's supposed to mean.

My take is to use alternatives with canon words as much as possible. In this case I'd just use koraavaan, although the -aan ending is in itself an interpretation of the sources we have (qahnaaraan - "defeated", spoken by the dragons at the Skyrim epilogue; mahlaan - "fallen", sizaan - "lost", etc.)