As for motives, since we have so much information on vpw's life and comments, we can trace a relatively clear path, at least in his own words.
We know that vpw told the early corps that he considered business, music AND ministry before going into ministry. (Yes, looks like he got into all 3, I know.) So, going into ministry was a CAREER decision, not a calling. We know that he claimed that in his first 2 years in ministry, each of those years, he seriously considered giving up. So, going into ministry wasn't about CONVICTION, but about convenience. We know that he took "Homiletics" or preaching, as his area of study- not "Bible history" or anything rigorous- he picked probably the softest option. When he was into his second year as a preacher was when he first heard the idea that the Bible was the word of God and that he didn't believe it prior to that. (We don't know what he based the first year's sermons on, but it wasn't on the idea that the Bible was really special.)
We also know that he inflated his credentials- like when he claimed he took ALL of Moody's correspondence courses, but according to their records, he never took ANY of their courses. (They have records of all completed courses, and there is NO record of him having turned in even one course.) We know that, around 1970 or shortly thereafter, vpw began claiming he heard from God Almighty back in 1942, in a promise that doesn't hold up to scrutiny. We know that, according to him, his first thought when he supposedly heard that was that EVERYONE WOULD LISTEN TO HIM if that was the case. We also know that he's cited snowstorms that didn't exist to punctuate events he later claimed were important. In 1953, he went to a convention where he met JE Stiles and learned from him, later plagiarizing the entire contents of Stiles' book on the Holy Spirit for his own publication. Of that day, he claimed the city was in a blizzard, and planes, trains AND buses were all out. (There wasn't even a single flake on the ground.) When confronted with this, he never claimed anyone was mistaken- instead he claimed that angels answered the phones at the bus depot, etc and lied to him each time.
All of that says quite a bit about the man, and that's hardly everything.