When is seeking help from other than God considered major polytheism that expels one from the faith, and what is the criterion for that, given that the one seeking help believes that the one from whom help is sought does not independently possess the power to benefit or harm, but rather through God's power granted to him, by analogy with the angels' ability to seize souls?

The criterion for polytheistic istighatha (seeking aid/succour) is seeking aid from a created being in matters that none but Allah can accomplish. And if the one seeking aid from the wali (saint/guardian) claims that he believes the wali acts only with Allah's permission, this does not remove the istighatha from the realm of shirk (polytheism); because it is considered a supplication to other than Allah, and supplicating to other than Allah is shirk.