Interesting you should mention this, because I've wondered the same thing. Of course, it raises the rather large "philosophical" question of how do we ever know anything about the inner life of another person. Everyone's psychic interior remains a secret.
As you say, Bukowski's whole thing is exaggeration in every way, so he goes to either extreme. My own feeling is that there is no doubt he was abused as a child--whether it was weekly, monthly, once a year I suppose doesn't really matter. Also some children are more sensitive than others, and even if had "only" been beaten five times in his whole childhood, if the events were traumatic enough and if he hadn't received any positive attention from his parents, then those events would be obviously magnified and internalized in a more horrific manner.
Again, my sense is that the abuse was likely extreme, because the trauma would explain many otherwise bizarre things in Buk's psychology. [the acne vulgaris and physical strangeness is another matter, but I don't think as central as the abuse]. The fact that he seemed to veer into a kind of limbo between reality and fantasy often means to me that something was shattered in his ability to trust the external world, to cope with reality OTHER than by "denying" it, taking refuge in imagination and fantasy, etc. There's a passage I think in Ham on Rye where he describes how bright the light was, as if the whole world was bathed in a kind of surreal, unreal, hopeless light.
So yes, he could "whitewash" and "blackwash" as you say--just different sides of the same character structure. As he could do those wonderful cartoons, and then turn and write mad and depressed, Dostoyevsky-style.
I haven't studied the matter and am no shrink--some "abused" kids perhaps grow up and are more resilient, less traumatized.
OK, I just remembered a poem which I came across the other day which perhaps says all this quite clearly [this is about the first half of the poem]
Bach, come back [from Bone Palace Ballet]
sitting in this old chair, listening to Bach,
the music splashes across me, refreshing, delightful.
I need it, tonight I feel like a man who has come back
from the same old war, death in life,
as my guts say not again, not again, to have fought
so hard for what?
too often, the only escape is sleep.
Bach saves me, momentarily.
so often I hear my father laughing, the dead laughter
of the father who seldom laughed in life
is laughing now.
then I hear him speak: "You haven't escaped me.
I appear in new forms and work at you through
I'm going to make sure that hell never stops for
then Bach is back,
Bach couldn't you have been my father?
nonetheless, you make my hell
I have come back from suicide, the park bench, it was a
but my father is still in this world,
he gets very close at times
and suicide creeps back into my brain,
sits there, sits there.
as old as I have gotten,
there is still now no peace,
and it has been months since I,
myself, have laughed
now Bach has stopped
and I sit in this old chair.