Pink, Using the ACT instead of the SATI is fine, but please bear in mind that you will still have to submit two SATII's.
Not playing a varsity sport will NOT affect your chances. Dance is an excellent EC for Williams. You should submit a performance tape with your application. See instructions under "peer and arts evaluation."
http://www.williams.edu/admission/ap.../Peer_Arts.pdf
Your dance resume which should centralize all information regarding your major EC: for example, a personal statement, a list of accomplishments and achievements, classes, an additional recommendation from an instructor or mentor, one or two (no more!) media articles about your performances
It's hard to tell from your very long list of activities what your #2 EC would be. You should condense and edit to focus on what you really care about (in addition to dance). Your essays and recommendations should reinforce favorite actitivies.