Is there sin upon one who entered the mosque while in a state of janabah, then prayed after washing the place of احتلام and performing wudu, and is he then required to repeat the prayer after performing ghusl for janabah?

No sooner had one become a major legal adult than emission became a natural occurrence for them, and what you did by not performing ghusl (ritual bath) afterward is insufficient. You must repent to Allah and make up those prayers you missed. As for entering the mosque while in a state of ritual impurity (janabah), the Hanbalis permit this if one performs ablution (wudu') for prayer. This is a strong opinion, supported by the actions of the Companions.