Como eu já disse anteriormente, o cânone do HHGTTG é… complicado.
Em termos mais simples, isso simplesmente não é abordado nos livros, mas o término do ajuste final da série de rádio oferece algumas dicas sobre o que pode ter acontecido.
In the final episode of the series, the babel fish saves Dent et. al. from destruction at the hands of the Vogons, calling them through the Whole Sort of General Mish-mash to the Restaurant at the End of the Universe, where Fenchurch has been waiting for Arthur at the end of time, and Trillian and Tricia McMillan are merged, essentially undoing the final step in the plan to destroy the Earth and all traces of it.
Isso não se relaciona com Elvis em si, mas indica que
the plan to eradicate the Earth and all its children simply failed, neatly resolving the problem of his continued presence at the Domain of the King.
Se os livros forem mantidos como a única fonte, a questão simplesmente não será resolvida, embora
And Another Thing... introduces new complexities and twists, wherein the plan again simply fails, and his continued presence requires no further explanation.