All other factors being equal, can one be intoxicated by alcohol if there is no observable HGN?

Not sure exactly what other factors you are taking into account, but these statements from a recognized source appear to make an explanation of why no HGN was observed (if any significant amount of alcohol was consumed) more difficult:

"An officer looks for the following HGN signs in each eye: • Lack of smooth pursuit; • Nystagmus at maximum deviation;and • Onset of gaze nystagmus prior to 45 degrees. These signs do not result from problems with the eye muscles directly. Rather,the brain and nerve centers that control the eye muscles are affected by alcohol,other central nervous system depressants,inhalants,or phencyclidine or its analogs.The signs appear in the order of testing as the level of impairment increases—i.e.,at lower levels of impairment, an officer might see only lack of smooth pursuit, but at the highest levels of intoxication, all three signs will be present.... the absence of nystagmus can protect the innocent or non-impaired driver who for some reason is unable to perform balance and walking tests but who has not ingested any alcohol or drug and is not impaired for driving."
APRI