Does the ghusl become invalid if the one performing it doubts having wiped a part of their head during the wudu' within the ghusl, then completes washing their feet without re-wiping their ears, knowing that the order of wudu' within ghusl is a Sunnah and not a condition?

It is from the Sunnah for the one performing ghusl due to janabah or menstruation to first perform wudu. Even if an error occurs in the wudu and a fard (obligatory act) is missed, their ghusl is valid if they cover their entire body with water with the intention of ghusl. Wiping the ears is part of wiping the head, and the order between them and the head is not obligatory. Wiping them is Sunnah according to the majority of scholars. The ghusl of the questioner is valid, and the prayer of anyone who prayed with it, without another wudu, is also valid.