The emblems are cumulative. If you get two of them, creatures you control will get +2/+0.

You can target a token creature with Sorin, Lord of Innistrad's third ability, but it won't return to the battlefield.

If a creature with undying is destroyed by Sorin, Lord of Innistrad's third ability, the undying ability will trigger, but the creature will already have been returned to the battlefield by Sorin. The undying ability won't do anything when it resolves.

If Sorin, Lord of Innistrad somehow becomes a creature, it can be chosen as a target of its own third ability. If Sorin, Lord of Innistrad is still on the battlefield when that ability resolves, it will then destroy Sorin and return him to the battlefield under your control.