The 5/15 players being mafia with powers is unfair. For instance consider what would happen if mafia had a framer who successfully framed a player n1.
n1: first player dies
d1: second player framed and lynched, (helped by the fact there are 5 mafia voting him up)
n2: third player dies, sheriff/invest is framed
d2: fourth player (sheriff/invest) lynched
n3: fifth player dies
and mafia is 1/2 of all players.