Is ghusl obligatory after surgery if ghusl is impossible for fear of harming the surgical site, and is prayer valid in this case?

Merely undergoing the operation does not necessitate ghusl (ritual bath). For someone who loses consciousness but does not experience ejaculation, ghusl is not obligatory according to the majority of scholars. Rather, it is recommended, due to what is authentically reported about the Prophet, peace and blessings be upon him, that he performed ghusl after recovering from fainting. Therefore, if you are not certain of ejaculation, ghusl is not obligatory upon you simply because of the operation. If semen is ejaculated and ghusl is not possible, then perform tayammum (dry ablution). Prayer is obligatory and does not cease as long as you are conscious, and repentance is obligatory for abandoning it, along with performing wudu and prayer according to your ability.