From my understanding chap's claim, not only does he choose up to four players to be potentially blocked, but he also chooses the number that will get blocked (not necessarily two).
I.E.
choose 1 suspect, 1 block --> 50% chance of blocking / 50% chance of blocking himself
choose 2 suspects, 2 blocks --> 33% chance of blocking both / 66% chance of blocking himself
choose 2 suspects, 1 block --> 66% chance of blocking one / 33% chance of blocking himself
choose 3 suspects, 3 blocks --> 25% chance of blocking all / 75% chance of blocking himself
choose 3 suspects, 2 blocks --> 50% chance of blocking two / 50% chance of blocking himself
choose 3 suspects, 1 block --> 75% chance of blocking one / 25% chance of blocking himself
choose 4 suspects, 4 blocks --> 20% chance of blocking all / 80% chance of blocking himself
choose 4 suspects, 3 blocks --> 40% chance of blocking three / 60% chance of blocking himself
choose 4 suspects, 2 blocks --> 60% chance of blocking two / 40% chance of blocking himself
choose 4 suspects, 1 block --> 80% chance of blocking one / 20% chance of blocking himself
It's actually quite a clever role and why I buy the claim. I don't know about blocking as many as possible as we're liable to screw up power roles. On the other hand, we'd have a bigger chance of actually preventing a kill and narrowing down the suspect list to the people blocked, and in which case we'd achieve the goals our power roles are supposed to accomplish anyway.
As for suspects, I'd say our daykiller is likely to be among the handful of people who dropped off in posting around 4-5 days ago right before shield's murder and the whole evil v. chap thing. I'm not so certain where to find mafia though, or even what exactly we're looking for given that they're supposedly dead.