So he preached like a Calvinist, but he obviously believed like an Arminian. His spirit bore witness that the preaching he did might actually influence a person and change a person's pathway from eternal damnation to eternal life. That is why he preached so tirelessly. He knew that it would have an effect on people's eternal lives. In essence his efforts were wholly inconsistent with his theology.
Your analysis of his motives might be improved if you were more familiar with his life. Benjamin Franklin (the deist) did not embrace the gospel but was so impressed by his sincerity in his task that he approached Whitefield about partnering with him to establish a new colony. But you know him better.
Calvinists realize they are speaking to people who are spiritually dead and will never be argued into the kingdom. They also know that God, in his wisdom has called Christians to present the gospel and warn people of the reality of judgment knowing that apart from the ministry of the Spirit no regeneration will occur.
I leave you with George's own words:
Salvation, every where through the whole scripture, is said to be the free gift of God, through Jesus Christ our Lord. Not only free, because God is a sovereign agent, and therefore may withhold it from, or confer it on, whom he pleaseth; but free, because there is nothing to be found in man, that can any way induce God to be merciful unto him. The righteousness of Jesus Christ is the sole cause of our finding favor in God's sight: this righteousness apprehended by faith (which is also the gift of God) makes it our own; and this faith, if true, will work by love.
Sermon 35. - The Conversion of Zaccheus. Sound theology from a man who didn't just talk evangelism!