I've seen what a seagull can do to a windscreen in the past, and after checking on the Californian wildlife there are some very large geese in California.
https://en.wikipedia.org/wiki/List_o..._of_California
Did any locals see any large birds in that area which may have caused a crash, or may have caused a medical incident if there had been a strike?
If the pilot knew the area well he would have known the heights of the surrounding terrain, and I would be surprised he descended so fast while IMC?. Seems odd he was climbing to then make a steep 4000ft/min descent from what I read above?
How old was the pilot? If he did have a heart attack or other medical problem would the post mortem find the evidence after a crash fire?