How to give crtitical feeback without sounding obnoxious?
First: Cromwell's Rule applies. You're absolutely sure that you're right, and they're wrong. That sureness should be a huge warning sign for you. So, accept the possibility that you are wrong.

Second: ask, don't tell.
So not this:


  You say C causes D, but actually D causes C!


But rather, something along the lines of:


  Do I understand correctly that you've assumed A and B and that you've then shown that C is correlated with D, and conclude that C causes D? How did you exclude the possibilities that D causes C, or that C and D have a common cause E?