I think Gautama took a very powerful Path in Tusita and had many attainments. It was not a Path of chance. I think He was beyond Stream-winner and even beyond a non-fully-attained Bodhisattva. I think He was fully attained in the Tusita Heaven, and He just came down to be the Buddha of our Era to restart the Path for many.
What’s left after Nibbana? There is no coming and no going, even in the Saha World. There’s no person even now, there’s no self to be found, no soul to be Eternally clung to. All is temporary, therefore for a fully Awakened One, even in Nibbana with residue, Death is not the gateway for the Higher Stream. I think those searching for Death are attached to form. They can also gain the Higher Stream in this life, and attain what is called the Deathless, without passing. This is my opinion.