I'm sorry to hear of your dilemma.
While I strive to give you an accurate and applicable answer to your question, and if I can possibly provide a legal avenue for you to have a successful legal outcome I will, I don’t have the ability to provide you with favorable law if it doesn't exist. As you read my answer to your question, I ask you to please don't shoot the messenger.
I'm afraid that you are on the wrong track in believing that by attacking the efficacy of the HGN (nystigmas test) in the presence of oncoming traffic is what is known as a red hearring. Even if you show that in your circumstance the test was administered in a faulty manner, it will not effect your case.
The test is based on the fact that alcohol affects the automatic tracking mechanisms of the eyes. Nystagmus is defined as "an involuntary rapid movement of the eyeball, which may be horizontal, vertical, rotatory, or mixed." Alcohol slows down the eyes' ability to rapidly track objects and causes to eyes to oscillate, or "jerk", before they normally would in a sober person. Alcohol stimulates the nerve endings, making nystagmus more pronounced in intoxicated persons. As a person's blood alcohol concentration increases, the eyes will "jerk" sooner as they move to the side.
A positive nystigmas test by itself is insufficient to prove you are under the influence. That fact would be proved with other tests as well--typically blood, breath or urine.
Here is a good site to learn about the HGN test:
If your situation is such that you are being charged with a crime based SOLELY on an HGN test, let me know and we can discuss this further. However, having the HGN test alone put in douby will not serve as a workable defense to driving under the influence and similar such allegations.
I wish you well.