How can we reconcile the permissibility of seeking help through God's mercy with Sheikh Al-Islam Ibn Taymiyyah's ruling that prohibits invoking God's attributes?

Seeking help through Allah's mercy is an act of seeking help from Allah and interceding with Him through one of His attributes, not a call to the attribute itself. The one being invoked is Allah, as in the Hadith (definition): "O Living, O Sustaining, by Your mercy I seek help." Similar to this is asking Him through His knowledge and power, and seeking refuge in His attributes. As for invoking the attributes themselves, such as: "O Power of Allah," or "O Mercy of Allah," or "O Word of Allah," this is forbidden and considered disbelief by the consensus of Muslims, because the attribute is not Allah Himself to be invoked; rather, it is subordinate to the One described, and supplication is directed to Allah, the Creator, not to any of His attributes.