Zoidark isn't dead. He's still alive. He's just not in the Lifestream proper and can't influence it or the Planet anymore. So he needs the Ascians around to do so.

Sundering definetly still works after death. There's a reason Ardbert exists. He's a bit of the Sundered Azem's soul; same as we are. Also, the Sundered Ascians are called Sundered for a reason.

We do know that the Sundered Ascians need to be "raised up" to get their memories back. Given everything we know about the Rift (specifically the Chrysalis), it's very, very likely that is where Zodiark really is. Given that becoming an Ascian involves leaving your body... it's got to be whack. I can't see Zoidark (or the Unsundered Ascians) just letting someone have the power that an Ascian does and not wind up Tempered by Zodiark. The last thing they need is some non-dying individual to take Hydaelyn's side and start wrecking the Ascians plans that way.

It's all but guarantied that we will be getting more info about what Azem did after Zodiark and Hydaelyn were both summoned. We have a very handy crystal of what Emet-Selch remembered of Azem on us as well as a power that can see past events when it is advantageous for us to see them. Azem's Crystal is a Checkov's Gun ready to go off and give us a very fun lore dump.

As far as the Echo goes... If Zodiark can Temper the Convocation... than he should be able to Temper Azem. Even with Azem not wanting to help summon Hydaelyn, I can't see them just letting Zodiark go around demanding aether via killing people. So... I can see Azem having to solve the "don't get Tempered by Zoidark" problem while having a lot of evidence that they would get tempered by Zoidark if they went up against them...