One of the names Julia writes in the cave is Peter David. David was a writer on the supergirl comics for many years.
The scene where Alex (Chyler Leigh) pokes Lena (Katie McGrath) with a stick was scripted, however, Katie didn't know when it was coming.
In DC comics the Kryptonian Demon is a deity as Krypton had its own Hell and demonic entities that were destroyed along with the planet.
The plot of a mystical dimension that Reign and Supergirl kept going to, in Reign's case when Reign would take over Samantha, bears a resemblance to what happened on the concurrent season of Legends of Tomorrow when the minds of Nora Darhke and Sara Lance would be sent to a mystical holding dimension when the demon Mallus controlled them.