Is a person who is afflicted by a Waswas (whisperings/suggestions) to commit a haram (forbidden act) sinful if he then fears Allah and refrains from doing it? And is his repentance considered invalid if he intended to commit a sin but did not act upon it, requiring him to renew it?

There is no sin upon one who merely thinks of a sin, nor upon one who intends to commit it then abandons it out of fear of Allah Almighty; rather, he will be rewarded for that. The servant is not held accountable for mere thoughts, as a mercy from Allah, for He has overlooked for this Ummah what they speak to themselves about, as long as they do not act upon it or speak it. Whoever intended to commit an evil deed but did not do it, a complete good deed is written for him. If he commits it, then only one evil deed is written for him. Therefore, repentance is not required for these fleeting thoughts and ideas, because he has not committed a sin.