A dead raccoon would beat Wilson in 1920
Pretty much, although the lack of whisper campaigns about his wife running the show post-stroke (stroke being butterflied) might allow him to at least maintain a core of supporters within his own party.
What would be TR's position on the League of Nations?
TR lived long enough that he had a position on the League as the proposals were taking shape. He had proposed an international peacemaking league several years earlier, so he was a fan of it in the abstract, but he was an ardent opponent of what he was seeing. The League united both hawks and doves in the Republican party in opposition: hawks because it might restrict America from acting in what it saw as it's national interest and imperil the Monroe Doctrine, and doves because it could force America to fight an unwanted war if the League called for it. Also it didn't hurt that it was being supported by a Democratic President and opposing it happened to be a political winner.
It's funny how people tend to look back at who opposed the League and automatically assume they must have been "isolationists" (ex. Henry Cabot Lodge, who was one of the greatest proponents of imperialism the Senate has ever seen).