Hello Riverbird...
I'm sure you'll get a variety of responses, but I'll attempt to answer your questions based on my experience.
Should I introduce myself as an addict in NA meetings?
In every meeting in my area there was once included in our "readings" something called a "Clarity Statement." The clarity statement requested all members to refer to themselves (if speaking) as addicts because NA focuses on the disease of addiction and to separate one drug from another would suggest that there are two diseases. So I would say, yes...if only out of respect. Some of the oldtimers in NA get their feathers ruffled when people introduce themselves as alcoholics or talk about "sobriety."
Should I avoid closed meetings?
I should say not! You've already stated that you consider yourself an addict because alcohol is a drug (a point that's cleary made in NA literature). I believe as long as you say you're an addict you qualify to attend.
Is it seen as a negative for an alcoholic to go to NA?
As a newcomer I attended both fellowships...and even to this day, I know of many alcoholics that attend NA meetings and many addicts that attend AA meetings. Many recovering folks I know say that as long as they can get to a meeting it doesn't matter which fellowship they go to. If there were no NA meetings where I live, I'd certainly go to AA. There's differences in our literature, our language and how we go about doing stepwork...but the 12 steps are the 12 steps.
G