Should one obey his mother even if she was wrong?

Obeying one`s mother is mandatory in matters that are beneficial to her and as long as she drive her children to commit sin.

Is it true that if a ritually impure, or a menstruating woman visits one who is in confinement, harm will be inflicted on the latter, or her baby?

This is completely wrong and unfounded piece of information.

Is Iddah (waiting period) incumbent on an old lady whose husband had died?

Upon the death of the husband, Iddah is incumbent on the wife regardless of her age.

Is it permissible to fish on Saturday?

All Perfect Praise be to Allah, The Lord of The Worlds, and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions. 

It is permissible for Muslims to fish on any day, but the sons of Israel are the ones forbidden to fish on Saturday. Moreover, there is no evidence in Islamic Sharia prohibiting Muslims from fishing on that day. And Allah Knows Best.