Taking nyquil on a regular basis can it cause the ethanol level in your blood to be significantly high and cause a positive blood test for alcohol?!

It depends. Nyquil is 25% alcohol. That's 50 proof. If you take enough of it, it will give a positive blood alcohol result.
Doubtful. Only if drinking in bottles quantity, and if is version still w/alcohol in it. If any concern, switch to non-alcohol formulation cold medicine. Also, should not need for ongoing use--so likely time to see your primary doc & discuss symptoms.