Some germ-line mutations predispose to cancer, yet often environmental factors (chemicals, exposure to radiation) are considered major risks for developing cancer. Are these conflicting views of the cause of cancer or can they be reconciled?

These are not conflicting views of the cause for cancer as "CANCER" is not a single disease with a single on/off state as many others are. Some cancers are more correlated to environmental factors (such as smoking and lung cancer) whilst other are correlated to germ line mutations such as BRCA1 and BRCA2 gene and breast cancer in females. The 2 causes mentioned in the question can aid doctors and physicians to look for ways in which they can help prevent or treat some cancer types (i.e. stop people from smoking, screen for BRCA1 variants in daughters of mothers with breast cancer etc. etc.).