I'm still baffled by your testing. Why are you testing a lens's focusing ability at infinity? Before you send it back, do this:

1) Tape a piece of paper with black marker writing on it (big letters) onto a wall. Put as much light on it as you can.

2) Set up a tripod about 10 feet away. To test autofocus, use the center point to focus on the paper. Disengage autofocus and turn the autofocus ring so that it isn't locked onto the paper. Zoom in, turn autofocus on, and focus on the paper again. Analyze the pictures to check for focus accuracy. (Really, you shold do this step several times to get a decent sample size).

3) To check for softness, do the same thing except use Live View x10 to manually focus on the target. Once focus has been achieved, take the picture.

4) Analyze results.