Is it the case that one who covers a Muslim so that God may cover him, or gives charity to purify his heart, or fasts for his prayer to be answered, is not considered sincere because his intention was not solely to seek God's pleasure, and is this intention acceptable to God?

There is no harm in observing good promises when performing good deeds, because Allah Almighty has announced them to motivate His servants. The Prophet, peace and blessings be upon him, approved Abu Talha's saying, "I hope for its righteousness and its reward" when he spent from Bayruha. He also encouraged maintaining kinship ties by mentioning their worldly benefits. What is forbidden is performing deeds for the sake of people, seeking their benefit or praise.