You are right that the examples are incorrect, you can't envy someone something, you simply envy someone. The "blah" comment is incorrect as well. To fix your examples you go as follows:

"I also don't envy you because of the probable consequences of this."

"Knowing the frustration he'll experience, I do not envy David one bit."

You envy someone, that's it. If you want to explain why, you need to add the reasons, but they will not be grammatically related to the word "envy" itself.