This is a super interesting question.
During ceremophosis, the host's body is completely destroyed, and in theory it can be returned using the spells True Resurrection (which can create a new body), Miracle, or the Wish spell, possibly Divine Intervention.

Technically, the humanoid body is completely destroyed, and the Ilythid body is a completely different person.

Does the caster need to destroy this illithid before casting True Resurrection, or is this not necessary?

Here's the most interesting thing, with the Emperor's case.

Let's assume that Ansur has found a powerful magician who is capable of bringing back the human Balduran, and is ready to cast the necessary spell.
But the Emperor, unlike all other Mind Flayers, has retained the identity of the host completely, and identifies himself as Balduran. Technically, it's still him, but not in terms of game mechanics.

What happens if True Resurrection is cast?
What if this creates a clone of Balduran if the Emperor lives?
If Ansur needs to kill the Emperor before casting the spell. Will Balduran remember everything that happened to him in the form of an Illithid after his resurrection?



Thanks to Larian for Baldurs Gate 3 and the reaction to player feedback