Is apostasy from Islam one of the things that invalidate ablution (Wudu')? What is the ruling for one afflicted with the whisperings (waswas) of disbelief and apostasy? Are his ablution, marriage, and prayer valid?

This trial is good for the believer, and what you find does not harm you as long as you are a lover of Allah, His Messenger, and His religion. Obsessive thoughts are from Satan and have no reality. Their cure is to remember Allah Almighty frequently so that Satan may depart, to ignore them, to fill one's free time with what is beneficial, and to supplicate much, seek refuge in Allah, and implore His protection.