What is the ruling regarding my ritual bath (ghusl) from major impurity (janabah) performed in the aforementioned manner, and is it valid? What is the ruling regarding past prayers if it was invalid? And is the ablution (wudu) for janabah valid for only one prayer?

As long as you have circulated water over your entire body, including hidden areas, with the intention of removing ritual impurity, then your ghusl (ritual bath) is valid, and so is your wudu' (ablution).

The ghusl for janaba (major ritual impurity) has two forms: a sufficient and valid form (which combines the intention with circulating water over the body), and a recommended form (washing the hands, then washing the private parts, then performing wudu', then separating the hair and pouring water over it, then pouring water over the right side and then the left). The wudu' performed during ghusl is like the wudu' for minor ritual impurity; one can pray more than one prayer with it as long as it is not invalidated.