Cassava flour and coconut milk

The only way to know for sure if those substitutes will work is to use them and see. I do not think the cassava flour will work in the pie crust, without other additional ingredients. However, it should work in the filling. The coconut milk could be used in place of the regular milk and heavy cream. However, it is likely to bring a rather pronounced coconut flavor which could overwhelm the pumpkin. If you are looking to use a non-dairy milk, might I suggest soy milk instead? You may have to add additional oil to make up the fat content, though. You could also use the coconut milk in place of the heavy cream, and by using the soy milk, there is less chance of competing flavors.