Must one re-perform the ritual bath (ghusl janabah) if, hours later, one remembers not having washed the external anal area, especially since water had previously touched it with the intention of purification from urine and not with the intention of ghusl? And what should one do if they forgot to wash behind the ears or under the armpits?

Performing ablution (mualah) in ghusl (definition) (ritual bath) is sunnah (definition) according to the majority of jurists, and obligatory according to the Malikis. It is waived due to an excuse, such as forgetfulness. So, if one forgets to wash a limb during ghusl al-janabah (definition) (ritual bath after sexual defilement) and a long time has passed, they should wash it with a new intention, and their ghusl is valid. However, if they prayed before washing the forgotten limb, they must repeat the prayer. Washing the forgotten limb before commencing the ghusl is not sufficient due to the absence of intention.