True, had Zain taken sides it would have made the situation worse which is something he tries to avoid. I remember in the past he has done the same because he knows how Suri can be and he probably saw for himself how Barkat has her wrapped around her little finger.
Shazia helping ZaYa would be great but a long shot, she is getting paid by Barkat and she does not like Aaliya so she wouldn't care. I think the only way Shazia would speak up is if Barkat does something to her son, which may happen based on spoilers but that has been floating around for a while now so its a matter of waiting and seeing how Barkat's track ends.