I think the mafia roleblocker would just be a normal roleblocker. It's my understanding of the definition that a day roleblocker would block roles that happen during the day, and since there are none of that here, and you specify they will be roleblocked during the night, it's just a regular roleblocker. (which would mean they could also choose at night who to roleblock, seeing as there's no need to do it before, because then you'd end up with a dilemma if they chose to roleblock the lynched person)
Another thing, I think definitely random.org is the way to go with fluctuating roles, seeing as I believe it would be possible for town to clear one another if they got to choose who to send the role on to. I also think that a role should be gone from the game if the person who's holding it is lynched, not just night killed.
Would it be possible for a person to be handed the same role several times in a row if random.org is used?
I'm also not sure if cop is too strong a role in this setup. Perhaps a tracker would be better, but idk.