If multiple creatures are put into your graveyard at the same time, Nether Traitor's ability triggers for each of them. Once you return it to the battlefield, you may pay {B} for the other abilities as they resolve, but they'll have no effect if you do. Even if Nether Traitor returns to your graveyard, it's considered a new object and won't be returned.

Multiple instances of shadow on the same creature are redundant.

Once a creature has been blocked, that creature remains blocked and will deal and be dealt combat damage even if it gains or loses shadow or if the blocking creature gains or loses shadow.

A token you own that dies is put into your graveyard before it ceases to exist.

If an attacking creature has multiple evasion abilities, such as shadow and flying, a creature can block it only if that creature satisfies all of the appropriate evasion abilities.

If Nether Traitor and another creature are put into your graveyard at the same time, Nether Traitor's ability won't trigger. This is because it must be in your graveyard before the creature dies in order for its ability that returns it to the battlefield to trigger.