My conception of a sit-in has no relation with anonymity. The important portion of a sit-in is the occupation of space. The space is occupied to disrupt the normal flow of activity, so as to draw attention to a cause. I'm not sure how a protest being anonymous goes from violent to non-violent.
Yes but for it to be a "sit in", the space had to be occupied by your physical body.
You could simply occupy space through other means such as placing a large heavy object in the store or place of business, but that would be considered 'dumping' and not a sit-in.
Secondly, it doesn't matter if Anon once DDoS'd minecraft.net. Operation Avenge Assange had its motivations declared quite explicitly: http://upload.wikimedia.org/wikipedia/commons/a/a4/Avenge_As...