Does Hajj become invalid if a pilgrim in a state of ihram experiences a wet dream and does not perform ghusl until the night of the tenth day, and are the missed prayers obligatory to be made up?

No, a wet dream does not invalidate Hajj, nor does it necessitate anything. One who experiences a wet dream may perform all Hajj rituals except for the circumambulation (Tawaf) alone, according to the majority of scholars. A ritual bath (Ghusl) is obligatory for prayer and Tawaf if semen is discharged. If no semen is discharged, then Ghusl is not required. Your Hajj is valid, and the lack of Ghusl does not affect its validity. When you experienced the wet dream and saw semen, it was obligatory for you to perform Ghusl and pray with complete ritual purity, or to perform Tayammum (dry ablution) and pray if you were unable to perform Ghusl. Your prayers are valid, and you do not need to repeat them. If you did not do so, then it is obligatory for you to promptly perform Ghusl and make up all the prayers you missed after the wet dream and before performing Ghusl.