In Nowhere, she corners Dahlia and Alessa in the final area, but Dahlia's psychic energy shoves Cybil back and knocks her out.
How did Cybill get there before Harry walk in?

Her own path, somehow or another. The cast are literally Nowhere, and thus there is no guarantee they're experiencing the same paths, rooms, hallways, and set-pieces.

I always found it amusing that Dahlia could incapacitate Cybil at will, but she lets Harry do his thing. Same for Kaufman. She should’ve been a psychic goalie and defended against the aglaophotis.

Except Harry didn't do anything, he was just standing trying to cope with Dahlia's revelation while God was being born.

And the reason why Dahlia didn't stop Kaufmann is clearly shown in the cutscene, because unlike Cybil he didn't waste any time talking - as soon as he arrived he shot her. There was simply no time for Dahlia to react. And after suffering from the gunshot wound, she is weakened and probably can't use her magic anymore. Hence, she couldn't stop Kaufmann from throwing the aglaophotis.