1- Madara Uchiha, Obito Uchiha and Nagato. It's also referred as summoning technique for both Madara and Obito, those are the only known users of this technique.

2- The first heart was detroyed by Kakashi with his lightning cutter, the second was using Hidan through his ritual, with Kakashi and Shikamaru's plan after taking Kakuzu's blood with a syringe, as for the last 3 Naruto and Yamato saved the day by killing the masks, with Kakashi finishing Kakuzu off.

3- as for Neji's death, everybody gave him two pots and the inside of them contains 3 flowers each, White roses represent innocence and humility