Yes the display could be the issue. Even though there may not be a dent or broken glass screen I saw one machine this past six months that had a screen with no dents or cracks but it did turn out to be the LCD only. Your previous steps were spot on but if Apple says the board / graphic card is good, the cable doesn't fix it, the LCD is all that's left. Did you have it fall recently? Just courious.
As always use safe static practices when inside your computer