Passives  can  still  activate  after  death,  like  Brakus'  passive  for  example.

Seeker  only  needs  to  be  hit  with  a  critical  hit  to  be  activated,  while  for  Sandlashed,  an  ally  needs  to  fall  below  50%  health,  and  I  don't think  that  includes  herself.

I am 99.99% sure that sandlashed passive triggers even if she is the one that drops below 50%. But, even if it doesn't - I am also 99.99% sure if I take one of her allies to zero in one hit, it won't activate.

I think the latter is correct, but the former is incorrect. 

Positioning of seeker and sandlashed will also impact passive. Seeker should be 1, sandlashed 4.

I think I understand now what is happening. Sandlashed's passive reads "...when any ally’s HP drops below 50%". I'm guessing that the game simply stops evaluating effects if the target dies. 

On the other hand, Seeker says "When critically hit ..." - which I guess is evaluated *before* the damage is done. IE, it checks if it is a critical hit, processes any events that occur as a result of a critical hit, and then processes the damage for the hit.

They  both  are  great  heroes though.