Bragi,
You made a big step, one that we all need to take with our parents. That took quite a lot of courage and resolve. Especially so as you held the line when your mother wrote back with a GGM (guided guilt missile) that for you deflected beautifully. You laid out the boundaries so very well - when she is ready to talk about her drinking.
I have seen too many people in my life, and some on this forum, state that they will not talk or meet with the alcoholic until they are sober and clean. That approach is a non-starter, and is underlain by the assumption that one can control the alcoholic and the alcoholism.
I hope you will stay firm for your sake and your family. You are showing great kindness to yourself, them and, believe it or not, your mother.