Is it sufficient to perform ghusl by placing a plug in the basin and filling it, then descending into it with the intention of ghusl, followed by rinsing the mouth and nose? And is the water considered stagnant, making ghusl in it impermissible? And is it permissible to complete ghusl in the same water when the length of the basin is insufficient to immerse the entire body at once? And does the water then become mustaʿmal (used)? And is it permissible to deliver water to the roots of the hair by immersing it without takhleel (running fingers through it)? And what is the ruling on the prayers performed with this ghusl if it was invalid?

Ablution from Janabah (definition) (ritual impurity) in a basin in this manner is prohibited, either as disliked (makruh (definition)) or forbidden (haram (definition)). However, the ritual impurity is lifted by it according to the soundest opinion of the Shafi'i school. Nevertheless, it is advisable to avoid it to steer clear of the differing opinion of those who believe the ritual impurity is not lifted, and to avoid the prohibition of performing ablution in stagnant water.

The ritual impurity is lifted whether the body is washed all at once or not, because the body of the one performing ghusl (definition) is considered as a single unit. It is not a condition to run fingers through the hair if the water reaches the roots of the hair. Therefore, your previous ghusls were valid, despite the dislike of the action.