No, the Sigma 35mm f/1.4 Art for Canon EF does not have full weather sealing. It is recommended to use a rain cover in wet or dusty conditions.
Yes, it works perfectly on Canon mirrorless cameras like the R5 or R6 when used with a standard EF-to-RF mount adapter.
The Sigma 35mm Art uses a 67mm front filter thread, making it compatible with standard circular polarizers and ND filters.