That is very interesting. I have never heard that going backwards on an elliptical machine caused knee problems. Going in just changes up the muscle groups you are working and going backwards helps change up the routine and prevents your feet from going numb (for those who have that problem).
You should go as fast or slow, and at as much resistance as you feel comfortable. It is important to make sure you have the correct posture on a machine too. That could be why some people experience knee strain but I love to go backwards on an elliptical machine and have never had a problem. I don't tend to lower the resistance, if that helps, but I usually exercise to random interval programs so it changes often. If you are having problems you can't correct maybe it is best if you do not. Good luck to you.