That the pattern of evolution is GODs plan. While I might not agree with this, it can definitely be in the realm of possibility. Is this the best explanation? Not at all, but it is probably the simplist. I am also very sure that Wiliam of Ockham did not mean for his principle to be used in this way. His way would be more of: How are were here through evolution or GOD? GOD because its a much simplier explanation. Part of why he said "God's existence cannot be deduced by reason alone."
I'm not quite sure I understand your point, but I will say a few things.
Biologists wouldn't use Ockham's razor to justify evolution. They would use the evidence that has been accumulated through experiments. However, they would use Ockham's razor to justify excluding God from the explanation. The point I was making, and you might be driving at this point as well, is that the proper use of Ockham's razor really requires a huge amount of knowledge about the subject in question.
As as example, take the question of human origins prior to Darwin's theory of evolution. Even in Ockham's time, there would have been any number of theories (not talking scientific theories here). Philosophers as far back as ancient Greece produced ideas very similar to Darwin's theories, but they didn't have the evidence to support them. On top of that, you have any number of theories, such as "We popped into existence five minutes ago". In this type of environment, it is difficult to apply Ockham's razor because you have a lot of theoretical arguments but no real way to assess the truth value of these arguments. In these types of situations, the apllication of Ockham's razor would mostly likely result in falsehoods because the only thing we could assess would be theoretical complexity.
Point being, if we thought real hard, we could probably come up with an argument explaining human origins that is theoretically less complex than Darwin's theory of evolution. The problem with this type of explanation is that it would not take into account the evidence we have accumulated through scientific investigations.
Also, while my knowledge of Okham is limited, I wouldn't be surprised if the quotation, "God's existence cannot be deduced by reason alone", is made in reference to the ontological argument.