there really are only two camps when if comes to Elaine:

That seems harsh.  She helped Harry numerous times in SK when doing so risked the wrath of a mad Lady, and she helped small time practitioners in WN that resulted in an empathy toward them that led her to helping found their organization.

The cave ceiling a thousand feet below ground level being full of tree roots that Harry narratively lampshades as being impossible under the conventional rules of nature is a pretty big hint there's something else going on with the cave. The stairwell down below and the barrier to the "minimum security" anteroom both pretty much melted out of previously solid material - that's plenty of opportunity for Alfred opening a gate so skillfully that Harry can't even tell it is a gate (another thing he's lampshaded the real pros being able to do).

I don't really buy that it's in the Nevernever either, though. Your reason that Mab was causing the long winter in Chicago by her presence in the cave taking care of Harry in GS is one solid point. Another is that it doesn't really fit with the surface reflecting onto some super-lethal patch of the NN, and putting supernatural supermax right in the Nevernever doesn't seem all that secure against some ungodly powerful creature of the spirit realm deciding to jailbreak its buddy by brute force either. Mainly, though, what seals it for me is that Harry didn't melt and die shortly after returning to Chicago because of all the cells that had been built on being fed intravenously by the tree roots reverting to ectoplasm.

I'm betting it's some sort of weird pocket dimension that Merlin made. Loosely related to how Bob said Harry could create a demesne within the NN if he wanted to invest sufficient effort, but Merlin's much more advanced technique let him put it in between worlds instead.

The Island does, the holding place of the things down below is not so certain to be in reality.

The books have been completely vague on this.  What makes you so sure?  Harry's home for example was protected from the Never Never side by Leah, without Harry's knowledge.  Why couldn't Merlin have been doing a lot of the work in the Never Never when constructing the Island?

The Never Never moves differently time wise.  Hades for example slowed it way down.  The prisoners are also held in a place where there are roots that are much too deep to be from tree's...  Perhaps the 5 different times at the same time were 5 different places within the Never Never where time is moving differently from one another.

The biggest issue I have with five Merlins in five universes at five times theory is that it makes Demonreach five times as vulnerable.

If it was created in one timeline, then it has to be unmade in that timeline.  But if it was made in five, then a successful attack on any of the others would destroy it.  That's five times as much trouble.

Five Merlins coming to make it in one timeline would at least avoid that.