I agree there is a massive difference between a hangover and withdrawals. I think the most key indicator that one is withdrawing rather than hung over is waking up with a desire to drink more. When I have relapsed after a period of sobriety that has cleared my physical addiction, like most normal drinkers the thought of another drink is the furthest thing from my mind.
Waking once the physical addiction has kicked back in leaves me wanting another drink more than anything else in the world.
I have no idea how decided the science is on the allergy question but I can see it is a very useful way to view things. It certainly functions much like an allergy in that something in us that we cannot control causes us to have a different reaction to alcohol than other people, so if you find that a useful way to view it, good stuff.
I personally have to drink daily for about three days before I start to get withdrawals rather than hang overs. The longer I drink for, the longer it takes to get over the physical cravings and shakes. After a recent eight-month long relapse of almost daily drinking, it took about six or seven days before I felt well again.
If only the physical addiction was the problem, it would be very easy to solve. I personally actually find it easier to abstain when I am in physical withdrawal because it keeps me so aware of what a terrible idea it is for me to drink.
I relapsed after over six years sober. That wasn't caused by a physical addiction. It was terrifying how quickly I became dependent again though.