A1) Unknown, but Naraku brought back Kohaku to life by using the shard of the shikon jewel in episode 29.
A2: Episode 20 of The Final Act. SPOILER: But Kohaku does not die because Kikyo had left a remaining light within his shard before he died. The light was assumed to be to kill Naraku, but Kikyo had planned to save Kohaku with it all along. Kohaku's shard was the final shard that remained, and after Naraku retrieved it, the jewel finally became whole again, leaving Kagome to be the one to purify it rather than Kikyo.