Say Demonreach was built on 5 parallel realities exactly the same except for the time at which Merlin lays down the foundations for the prison. The foundations of the prison are done at different times in reference to a standard timeline. But they are synced such that the spells interlace, resonate at nearly the same time.
To undo Demonreach, one option is to go back in time and prevent Merlin laying down the foundations at anypoint in time or parallel reality. I don't think there is enough power to undue this act as you first have to fight against conservation of history, then you have to fight against metaphysically significant events, and then you have to fight against Odin's admonition of the high price of traveling back in time to change the past with huge unknowns resulting in such an event. There also would likely be huge temporal waves warning people of the impending attack. So, that wasn't what occurred.
Next, what likely happened was something in a close to parallel reality to our Dresdenverse time that got attacked. Since Demonreach was created in say 5 times, I think their is a clock for each of these times/parallel realities starting off at the point of Demonreach's creation. In other words, five parallel worlds are/were perfectly aligned time wise at the moment of Demonreach creation. So when 200 years passes, time on each of the parallel worlds could be 600AD, 800AD, 1000AD, 1200AD, 1400AD, 1600AD depending upon the date of it's creation. So since Demonreach was attacked in our reality in 2014ish, it would need to be attacked in the other realities on staggered dates in the 5 realities in total possibly all at once.
It always bothered me why Maeve and Lily waited to do their assault until Harry was alive and kicking. It never made any sense. Why not do it while Harry was a ghost (I get it, Mab was on the island), but they've had years to do it from BEFORE Harry was even the Warden. I've always kind of assumed that Harry claiming Demonreach and personifying the island's genuis loci made it stronger (cuz now it has a Warden), but also made it vulnerable, since now it's a physical thing that can be assaulted, if that makes sense.
Even if you accept that it had to be done on Halloween (because Maeve also wanted to kill Lily, or the attack required it for some other technical reason), there were several Halloweens' worth of missed chances, and a few shots at it before Harry was awake.
Therefore, I propose the following:
First, I think Demonreach had to be awakened in order for it to be made vulnerable. So Harry had to claim Demonreach in Turn Coat before the Cold Days attack was even possible. Next, I think Harry's unique nature of being Starborn, Warden, and being vulnerable to Winter Lady is what was the pivotal pieces Outsiders needed before Cold Day's attack could commence. There are other likely required setups that needed to occur. I like the idea that the Cold Days attack had already attacked and was completed at least in one reality already, so the main events were fairly set in stone. So, the Cold Days attack we viewed was in a staggered parallel reality where a 'future' Cold Days plot had already occurred in our favor but may have played out in some ways differently. This makes in some sense our Cold Days a clean up operation and gives new light on Vadderung's comment to Harry "Maybe you already have" in answering Harry's question "Can I do this?"
This then allows for plots in some other staggered realities to occur. But, I think it's fairly safe to assume that once Demonreach is safe in one parallel reality, it likely is in all unless the Outsiders attempted to redo the attack by traveling back in time or choosing a different parallel reality to make the push into. (That is if Demonreach can even be undone by attacking it in only one parallel reality.) If it can't be undone once an assault fails in one staggered reality, then the Outsiders plan isn't really the destruction of Demonreach after that but to maximize the destruction short of it, like having the Warden release all of it's Prisoners voluntarily by having Maeve command Harry to release them.
As to PG and Cold Days, I think there will end up being a link between the two and another book will start tying each and it together. I think we will learn more about the importance of those individuals that ended up at Arctis Tor in PG. Murphy, Molly, Charity, Thomas, Lash, Lasciel via Lash, Harry. We already know the importance of Molly and Harry; though we will likely learn more.
