I would hardly call La Vey an expert, all his 'knowledge' is recycled and degraded by his lack of intelligence and need to be a shocking religious leader/con man.
Agrippa's renouncement is a strange case indeed, it has been blown out of proportion, he states his first publication of his occult philosophy had erroneous ideas in, and ideas which went against his later more 'Christian' attitudes. But there is evidence to suggest he edited it heavily and actually published it once more after making his statement that he renounced his works, perhaps meaning that he rectified it. There are some people who draw the conclusion that Agrippa rejected magic in it's totality, others state he rejected some things and not others, others state he rejected nothing personally, only publicly. Either way, the circumstances surrounding this event in his life are sketchy and lost to history, we only have interpretations at this point. But why he would push for a work he no longer believed in to be published when he was close to his death is beyond me if he did not feel it would be useful.
I had no idea Levi renounced magic, I would love to hear the source for that. It is news to me, but I don't know a lot about the man's life in all honesty. I love his works though.
One thing everybody needs to learn about all occultists is that what we say in public is not necessarily what we feel and think. I will often tell someone a blatant lie because they would believe that over the truth and would arrive at the truth of the matter all the same. If I do that, if people I know do that, why would these 'experts' not do that? Especially if there was some motivational factor driving them to be left alone which simply hasn't been reported.
Either way, we will never have any idea, I do know Levi and Agrippa had great minds and have contributed a lot to the Western Mystery Traditions and all magical traditions of the West, including witchcraft.