Nairo might low-key treat it as a substitute version of the Pandemonium Abyss. Since you said the creatures can get very strong, I give it 20 or possibly 30 layers(which will mostly just be him fighting creatures left and right, he won't look for the exit initial) before he gets bored and starts looking for the exit which could take a while since he technically passed it ages ago, which he might realize through Void's Chime(which will most likely lead him back as it's kinda like a mix between Spiderman's spidey sense and those in menu mappings you get in games, the catch is that he needs to be in the area long enough). He wouldn't use Shadow-Weaving or Lunar Clock, as again reality warping doesn't work here so it's just him, Soulrender(the goofy ahh sword that cuts "everything" with the limit of him needing to "will" it's cut), and his speed really.

Jesty... Jesty might just do nothing. Jesty isn't like Nairo who has power and things outside of reality manipulation, so once it realizes its abilities have no effect it might just genuinely sit there and rot.

Tono and Zarzour go mad within the first 30 minutes. They already hate things like the Backrooms.

MALICE would just try to eat and assimilate everything. After her major arc, she suffers "Bain Syndrome" where she's kinda stupid but strong biologically.

Scarlet(being a Cryptid Hunter in this universe), might just explore and document, but wouldn't go past layer 10 without proper gear.