It is said that the liberated souls reach Srivaikuntam when they shed their mortals, this abode is eternal and in the purest form. If so where is Thiruparkadal? Is it a permanent abode as well? What is it made of? Though Adisesha is a nitya suri he cannot be a vibhu like Perumal; so how could he serve as a recliner at both the abodes of Perumal simultaneously?

While it is possible for the liberated souls to take desirous forms when they reach Vaikuntam and it is absolutely possible for the Nityasuri like Adishesa to render services to Perumal same time at various places.
Ksheerabdhi is situated outside of Srivaikuntam where all the demi-gods like Brahma, Rudra go to worship. This place is formed by the divine will of Perumal and it is not created by Chaturmukha Brahma.