I'm going to offer some thoughts, I won't dignify it as a wag. One is a matter of semantics. Demonereach was built and I quote.The semantic issue arises out of the arrow of time, build a thing in the past and it exists in all time going forward. If on the other hand you build it in the future and want it in the past then you have to build it backwards. It doesn't make sense otherwise.
The second note is about who could know of those symbols. There are two sets of characters in the books who could know. Angels and the Archive. Since the Archive contains the body of knowledge of the human race, if the symbols were used by humans she knows and understands them. And of course angels are know it all's.
A final note. If the Oblivion War is meant to wipe out all knowledge of the things that live in the prison then you have to accept that the prison will eventually need to pass out of human memory. That implies that there will be a final Warden.
Well, one could argue by building it in 5 different time periods simultaneously, he was building it both forward and backward at the same time (depending on his perspective or the perspective of anything watching).
From a physics perspective, I'd say he was building it outside the normal passage of time relative to the universe (obviously not relative to himself). A bubble of separated time, if you like. This is why he would appear to be in several points of space-time simultaneously (which of course is impossible in physics). In fact, I suspect in relation to himself, he was building the prison just the once. Kind of like being in the centre of Venn diagram made of five circles. He's in all yet also in his own space. A space only possible due to him being in it and linking the five other spaces.
There are others apart from the Archive and angels who could have known what those symbols are - gods, higher-level demons, certain faeries etc. I suspect many such powerful beings could know that sort of thing. Also, any wizards who learned Merlin's magic (or even his predecessors) could well know his stuff potentially. Harder to say though.
Is the Oblivion War meant to wipe out knowledge of what is in the prison? I might have missed that. I didn't really see the two being related myself.
I will point out that the symbols are symbols and as such part of the human lexicon. If the Archive doesn't know them then she isn't what Jim says she is.
Well, the point of such a archetype like the Archive in the story apart from having them as an information source/dump is also that their information has holes/can be wrong. It really helps with those sucker punches that Jim never uses...
The Archive has sort of changed a bit over the series. At first she was just the repository of all human knowledge that had been gathered by her predecessors. Then she became the repository of all human knowledge ever written down (even if it were later destroyed, it was now embedded within her). First she simply used to learn things like any other human. Then she was able to learn anything that had been written by anyone, anywhere.
Of course...not all knowledge is recorded. So one enormous gap in her knowledge would be any oral knowledge. She also would not have access to information that was only in people's heads. That's a few big gaps already.
