What is the ruling on writing the shaddah above the fathah in Quranic verses, and is it obligatory to seek out the one who did that to advise him, and is a person sinful for not offering advice while knowing about it?

Writing the shaddah above the fatḥah changes the fatḥah to a kasrah in modern orthography, altering the shape of the letter, and this should not be done intentionally to avoid ambiguity or error. However, if it happens unintentionally, especially due to electronic writing, it is excused based on the ḥadīth, "My Ummah is forgiven for mistakes and forgetfulness." There is no sin upon the questioner for not advising the writer. As for the second phrase, if it refers to the fear of shirk and idol worship returning to the Ummah, this will indeed happen near the Hour, as stated in the ḥadīth.