Are the proceedings of the State Legislature considered valid even if it's later discovered that some ineligible person took part?
Yes, the proceedings of the State Legislature are considered valid even if it's later discovered that some ineligible person took part. The answer can be found in clause (2) of Article 189: "A House of the Legislature of a State shall have power to act notwithstanding any vacancy in the membership thereof, and any proceedings in the Legislature of a State shall be valid notwithstanding that it is discovered subsequently that some person who was not entitled so to do sat or voted or otherwise took part in the proceedings."