Do prayer become invalidated and apostasy occur due to the whispering of slandering Allah and slandering wudu, or doubting the number of rak'ahs, or other obscene thoughts and phrases that come to the one afflicted with obsessive whispers?

You should disregard whispers (waswas) and pay them no mind, as they do not harm or affect the soundness of faith or prayer. It is obligatory to ignore them and not give them any attention until Allah grants recovery from them, no matter how often they recur or how varied they become.