Is it obligatory to have the intention for ablution before commencing it, and is it legislated to say: "I bear witness that there is no god but Allah, He is alone without any partner, and I bear witness that Muhammad is His servant and His messenger" after ablution and before water covers the entire body in ghusl from janabah?

Ablution (Wudu') before performing the ritual bath (Ghusl) is not obligatory. If it becomes invalid, it does not affect the validity of the Ghusl, as long as the intention is present and water has been circulated over the entire body. The intention for ablution before the ablution for major ritual impurity (Ghusl al-Janabah) is not required; rather, it is considered commendable by some jurists. The mention (dhikr) recited after ablution is commendable, not obligatory. We are not aware of any jurist who mentioned its commendability after ablution during Ghusl al-Janabah. However, the Shafi'is mentioned its commendability after Ghusl, just as it is after Wudu'. It is advised to avoid obsessive thoughts and to consult a psychiatrist.