No, Extra Channeling does not grant 2 more channels to each type of channeling. See page 159 Sidebar of the Advanced Class Guide entitled "Feats that grant extra uses". Essentially, it gives you two extra uses that you can spend on any type of channeling that you wish.
Yes, Channel from Life Spirit and Rebuke Spirits from Spirit Warden archetype both get their own pool of channels, and if you have Extra Channeling you can apply it where you need it each day (but it only gives you two extra channels, not two per type).
No, Channel and Rebuke Spirits don't stack for damage. You can use Channel to heal or to harm undead as a Cleric of your Shaman level (1+Cha mod times per day), and you can use Rebuke Spirits to harm undead as a Cleric of your Shaman level (3+Cha mod times per day). The damage doesn't stack.