To what extent are the hadiths valid that state the universe and all creatures were only created for the sake of Prophet Muhammad (peace be upon him), such as the hadith of Gabriel addressing him, the hadiths of Adam seeking intercession through him, and the hadiths of his creation from the light of Allah?

The aforementioned hadith is fabricated and has no basis in the books of Sunnah or even in the collections of fabricated reports. What indicates its fabrication is the statement that Gabriel's name was not known until after ten thousand years.

It has been previously explained that the hadiths which speak of the creation of Muhammad, peace be upon him, from the light of Allah, or the creation of all creatures from his light, or that but for him the creation would not have been created, or Adam's seeking intercession through him, are weak and fabricated.