Yes definitely I highly agree there. If alcohol is causing things to get worse - and it's your house - then remove the alcohol. It might not be a 100% solution but it's at least a start.
Yes tell people up front that it is your request for this time. Maybe get a bunch of different teas, instead, and have it a dinner with a variety of herbal teas? That way people have something interesting to drink and that becomes your new tradition.
Thanksgiving is awful for many families because it's a time for people to get together and start thinking about what they are thankful for - meaning they sometimes think about the things they want but don't have. So one mom might start harassing her son about why he has a sucky job and doesn't have a better one. Another mom might start harassing her daughter for not being married. There are countless things parents harass their kids for. "Why don't you visit me more often?" "How come you don't take me on a cruise, my friend's kids take her on a cruise every year." So this is a very common problem people have to face. I have a friend who is gay and 22, and her grandparents hassle her *constantly* about settling down with a guy.
You can't really change them, you can only look at it as a personal growing challenge, to learn patience and inner strength

One option is to have a private Thanksgiving, if it gets too much. My mom tries to get me over there every Thanksgiving. My boyfriend and I have Thanksgiving at home, with just my dad. It works out well.