A simpler solution would be to reload the page passing a parameter that you want to use other CDN; something like this: https://gist.github.com/4444636
You've now got people wondering why their page just reloaded after half a second and a slug of unnecessary javascript in your page all for the sake of simplicity.
A simple solution would be to just link to a self-hosted version of the lib.
False; because (if the script is inside the head of the page) the loading of Javascript is synchronous so the user will not notice any refresh at all.