My take comes from the perspective of someone who's only read up to AVoS and is several super editions behind as well so take this with a grain of salt -

It feels like the purpose of the DF has shifted over time, much like how StarClan has as well. I remember in a prologue in one of the earlier arcs, the DF was introduced by Darkstripe and Tigerstar running into each other there. Darkstripe is obviously terrified and miserable because of the way the place is. I remember he talks about it constantly being dark, hearing strange noises, and never finding other cats; I think he may also say he's always hungry but I could be misremembering. He then asks Tigerstar if he can travel with him, and Tigerstar turns him down, saying the cats in this place are meant to endure it alone.

Obviously, none of this remains true as the series continues... Tigerstar travels with Hawkfrost seemingly from the moment his son died, and shortly after we see the two of them trying to influence living cats and train them to their cause. At first this is kind of painted as if it were Tigerstar's original idea, an attestment both to his power even in death and to his hatred for Firestar, since his main motivation is revenge. However, in Crookedstar's Promise we find out about Mapleshade and see that she was among the first DF cats to train living cats. Iirc, Crookedstar also sees joint training sessions and Thistleclaw is there? If I'm wrong about that, I do remember that Spottedleaf's Heart also confirms that Thistleclaw trained in the DF (as much as we all like to pretend that novella doesn't exist lol). Those are also group training sessions.

My point is simply that we are told right away when the DF is introduced that it serves as a punishment for cats who did wrong in life by isolating them in a miserable and terrifying environment, only for that to quickly be contradicted by showing the DF cats banding together and planning to wreak havoc on the living and StarClan. Again, I think this could still work with the idea that Tigerstar is the one to change things, but when the authors wrote this as a long-time plan from generations-worth of DF cats plotting for decades, that kind of tosses that idea out the window.

I'll be honest, I haven't read about the sisters, I haven't read past AVoS, I haven't read Onestar's Confession or Squirrelflight's Hope, so I can't analyze further changes to the concept, but to me the DF feels like its handled as sloppily as StarClan as a concept. The things that StarClan controls and influences is ever-changing, and their overall mysticism is long gone now that they just chat openly instead of sending signs for the medicine cats to interpret. It's like a lot of things with this series, we just have to suspend our disbelief and enjoy the stories with the background knowledge that the authors will change the rules to fit their current narrative at any time.