If a person died claiming to be a shia Muslim – but committed major sins till the end of his life – eg drank alcohol which actually caused his death due to liver disease.
Then 1) is it permissible for his family to do a majalis (of Imam Hussain A.S) for isaale thawaab for him?

2) Should one attend such a majalis

3) Also, should one offer condolences to this person's family?

Yes to all of these questions and we should pray for his forgiveness.