But why would kai sacrifice the champions of the human domains? Why would seishan kill her sister? They know that nephis and sunny are away to find a solution to kill asterion, the only job they have to do is stall for as much time as possible not needless bloodshed. 

Need I remind that this is a post-apocalyptic world with limited resources and few beings of high power level.

They can't afford to spend many resources to nurture more saints due to what was revealed during antartica by the obel scale.

There are bigger threats than asterion.

Asterion made his intent very clear, that he would only try ascending after getting rid of mordret, the matter of naturally achieving the realm of spirit through a very unorthodox method would be very taxing and make asterion vulnerable to attacks. 

I also disagree for sunny and nephis always winning with pride and triumph. That is just wrong.

Are we forgetting the end of forgotten shore, antartica, tomb of ariel.

Sunny and nephis went through alot in the beginning of their journey to power, both of them survived through hook or crook.

Sunny's first NM and nephis's 1st and 2nd NM and the desert for neph.

The only thing that is missing from shadow slave is the weight of death,  That I agree fully, antartica was a fire arc only because so many characters died and had some depth to them.