Yes, it is a professional L-series lens featuring dust and moisture-resistant seals, though a front filter is recommended to complete the seal.
No, the EF 50mm f/1.2L USM does not have built-in optical image stabilization.
Yes, it works perfectly on Canon mirrorless R-series cameras using the EF-to-RF Mount Adapter with full autofocus support.
The lens uses standard 72mm screw-in filters.