If you want to understand the situation of pre-Buddhist bon than read Drung Deu and Bon by ChNN.
Not if many Böns had become corrupt back then.
From what I've learned there are basically three types of Bön:
"Black" Bön is apparently Shamanism more or less devoid of the Dzogchen of Yungdrung Bön.
"White" Bön is apparently Bön mixed with Buddhism mainly for the purpose of avoiding persecution.
Yungdrung Bön is Eternal Bön which has the pure Dzogchen Transmission, such as the Zhang Zhung Nyengyud.
It would have been "Black Bön" that would have fallen into some nasty sorts of practices, corruption, etc.
But I believe that all three forms of Bön have Shamanistic practices.
It seems that these days most Bönpo's teachings are a mixture of White Bön and Yungdrung Bön.