Is ترتيب (ordering) obligatory in ghusl (i.e., the head, then the neck, then the rest of the body), and if this order is not followed, is the ghusl invalid? And what is the ideal method to avoid extravagance in water during ghusl while ensuring water reaches all parts?

The order among the limbs is not among the obligations of ghusl. Ghusl is achieved by intention and by conveying water to the entire body. It is preferable to perform ghusl with the sunnah description narrated from the Prophet, peace and blessings be upon him. Excessive use of water is blameworthy, as the Prophet, peace and blessings be upon him, "would perform wudu' with one mudd and ghusl with one sa'." Therefore, one must strive against the self to abandon extravagance and emulate the Prophet, peace and blessings be upon him, and close the door to obsessive doubts.