if Nadal even plays Roland Garros (I am coming to terms with the idea he might well already be retired), there is no way in hell he is seeded outside the top 4, no matter where he is ranked. I would be willing to bet money on that. They even seeded Venus Williams, Venus clay court guru Williams at #4 when she was ranked much lower than that in 2004, out of respect for her past record. She had even done poorly at the previous years RG and doesnt even have a good RG record, it was a respect for her overall stature in the game I guess.
Nadal as the all time greatest RG Champion would not be seeded below David Ferrer. He would be at minimum seeded #4. I would not even be surprised in the least if they seed him #3 over Murray given Murray's modest past record at RG, if Murray is still only ranked #3 by then.