Is ghusl obligatory if, some time after performing ghusl for janabah, one finds a dry substance on the penis, while being uncertain if it is semen and without having felt it exit? And what is the ruling on prolonging ghusl for janabah for two or three hours?

You are not required to repeat the ghusl (ritual bath) if you are unsure whether the dry discharge is semen or not. You are also not required to perform ghusl even if you are certain it is semen, because if it is a residue of previous semen, many jurists do not consider repeating the ghusl obligatory. And if it is new semen that was discharged without passion, it does not necessitate ghusl according to the majority of scholars.

As for prolonging the duration of ghusl, that is a form of waswasah (whispers/obsessive thoughts). The lawful ghusl only takes a few minutes.