I don’t have definitive answers, but I can answer with my anecdotal experiences and the answer is no for both. Cascade is slightly better about collision than Niagara at present but nothing works that great. Can’t even get kill on collision or stop moving on collision to work.
I don’t know how granular you need the rain to be for the player, but I think volumes might work best. You can even create them dynamically with line traces from above. Maybe even physics actors locally around the player? All depends on how much you need to do at a given time with rain, but it might be best to try to fake it while Niagara is still under development. I doubt Cascade will be getting any significant updates any time soon.