Point #1 - I only know a little about the Judeo-Christian-Islamic traditions that assume a moment of divine revelation. With the exception of certain creationists, I don't know of any religious people who would deny that human beings predate the holy books.
Point #3 – I don't know of any religious books or educated minds that would deny this fact or pin their faith on the immutability of language.
Point #2 – this seems to be the crux of the issue. Is language complete? What does that mean? What is the assumed status and significance of a "holy book?" Is language an effective (complete) medium for capturing the content of these books?
Some facts to take into account:
In the form of great poetry and literature, language can be beautiful and inspiring. Great change and powerful emotions can be effected in the space of just a few syllables. If religious texts are a form of divine literature, if the goal of a holy book is to inspire, precipitate change, and instruct about observance and morality, than language is a reasonable medium for this content.
There may be some philosophical difficulty in trying to articulate some ineffable and immutable truths about infinite and transcendent beings through the medium language. But the philosophical problem would be to define the terms: “ineffable, immutable truth," “infinity" and "transcendence” in this context.
You may or may not care for it, but some people have dedicated their entire lives and everything they own for the sake of poetry and beautiful literature. We tend to think of language as a clunky and imprecise medium for conveying facts, but when implemented artfully, the power of writing is often best described in pseudo-mystical and pseudo-religious terminology. Why should language have this power over human thought and human emotions? Is it a good thing to be inspired in this way? These are valid questions, but irrespective of their answers it is not trivial to criticize the implementation of religious language in religious contexts.
Appendix – Examples of poetry characterized in mystical and religious terminology:
The famous postscript to Howl (It starts 3 minutes and 26 seconds into this video):
"The typewriter is holy, the poem is holy, the voice is holy, the hearers are holy."
Jorge Luis Borges – Salvation by Deeds:
It is true. They have thought up that
atrocity, but there is also this
something quite different, which fits
in the space encompassed by seventeen
The divinity intoned them. They were
in an unknown language, and I could
not understand them
The leading divinity delivered a
Let men survive.
Thus, because of a haiku, the human
race was saved.