Is what I do — as a Ja'fari youth who prays and fasts with Sunnis, and celebrates only religious occasions, without reviling the Companions or the Mothers of the Believers or self-flagellation — considered a flaw, or polytheism (shirk), or a departure from obedience to Allah?

A Muslim should be insightful and guided by his Lord. It is his right to rejoice at the birth of Ali (may Allah be pleased with him) and to grieve at the killing of Hussein (may Allah be pleased with him). However, the problem lies in the corrupt beliefs, reprehensible innovations, and occasions and celebrations for which Allah has sent down no authority that result from this.