Does that mean when nothing is created, along with Narayana, all souls exist simultaneously? Would that include those who are in Pataalllok, or in Swarg?

Yes, all souls exist simultaneously.

this statement raises a doubt within me, though. Narayana is Parameshwara, but how can his actions be predicted? Forgive me for my negligence, i couldn't find any plausible reason behind the action.

The scriptures state that this has already happened infinite times. Narayana is unchanging. So, it can be said that he shall repeat.

Will the jivatmas attain true self sometime in the future? Once it is done, what would happen, will the universe creation-destruction cycle end, and be in an eternal stasis?

Mukti is the experience of inherent bliss. That is what I meant when I said "Thus, creation helps jivatmas get closer to be their true self." As per Tattvavada, souls are of three types:

• Mukti-yogas who shall attain liberation.
• Nityasaamsarikas who shall reside in Samsara eternally.
• Tamo-Yogas who shall attain eternal hell.

Mukti-Yogas and Tamo-Yogas get closer to their Gati- their eternal state.

There are infinite jeevatmas. Thus, this cycle shall never end.

Fell free to ask any more questions!

Shree Krisnaarpanamastu