I posted on January 16 about problems integrating Drupal 7 with an external website. I had to set it aside and work on other things until this week. I finally located the cause of the problem today. As are most time-consuming problems, this had a simple solution. I just had to step away from it long enough to reset, then look through the code with a fresh outlook.
The culprit in this case was running the Drupal login function targeted at http://www.mydomain.com, but then trying to retrieve the Drupal node from http://mydomain.com. Since the domain didn't match, my session auth failed.