https://docs.google.com/spreadsheets/d/1Jpq6MlF72Fg4kPlCCtorJ5ClNA0P8n7IJlKUPckidxg/edit#gid=5779210

This spreadsheet has 700 recorded plays of Anata to Risou no Heroine with both Base and non-Base Crit Rate skills. The results heavily support the assumption that all Skills are additive.