The author of this passage is referring the characteristics of the writer of the novel. He is saying that the writer of the novel has "Inventiveness. Wit. Artistic integrity."
This is an introduction to the novel so you would expect the writer to talk to some extent about the author of the story. It looks to me like you are interpreting the first statement correctly.
"To say them, however, is perhaps to talk more about the mind behind the words than the ends to which they are addressed." It means to say those three things, "Inventiveness. Wit. Artistic integrity," describes the mind of the author (PKD) more than these words describe the novel itself.
The second sentence, "For to say them in all good-intentioned honesty about a story results mainly in a heaping of abstractions." confirms to me that he is talking about these words describing the author's mind more than they describe the novel he has written. He is saying that when these words are used to describe the novel the result would be pouring on somewhat meaningless abstractions when applied to the novel. "The novel is witty." Blah... The novel is "inventive." blah, blah. The novel has "artistic integrity." blah, blah, blah... He is saying these words are abstractions and don't really tell us much specifically about the novel if used to describe the novel but if used to describe the mind of the author of the novel these words are more powerful.
This is how I interpret those two sentences. I agree with you that they are written somewhat obtusely. Roger Zelazny probably could and should write those sentences more directly and clearly but maybe he has a purpose of trying to show that he has "a way with words." Who knows. I agree the two sentences are difficult to interpret. But I think you got the basic idea although you probably should not have had to puzzle over their meaning and put this much effort into understanding them.
And then again, maybe I am completely misinterpreting the two sentences. I'm taking my best shot at them. Thanks for sharing them. He won't get the prize for clear, precise, easily understood prose. ;-)
Bill