The book: There are several books published with the name.
The author: Here's the best that I can do:
The sigils: Do you mean the Elder Signs? They've been developed.
The demons? They're real now. Come to speed please.
I'm not aware, after reading some things about Lovecraft and his writings, what occult symbols, rituals, or techniques ("deeper occult symbolism" with which his work is "entrenched") Lovecraft really knew about. Could you refer us 'kiddies' to sources who detail this? Joshi? A title? Was he aware of Theosophy secrets? Lesser Banish Rituals of the Pentagram? Do tell!
Your interest in portraying all of what has been constructed in association to the Necronomicon as fiction isn't convincing. Numerous occult authors are making use of Lovecraft's fiction, and some of these are serious attempts, even using the title 'Necronomicon' to make useful arcane tools. Those are not fictional.
What convinces you that fictional gods, demons, or elder gods cannot be summoned? Why would they have any different relation to reality than other demons? I suppose that they do not, and your contention should be followed up with some method to show how you can confirm the 'reality' of the demons (or gods) outside of Lovecraft's stories.