So, let me take a stab at your questions.
If all experience is constrained and undermined by language, how can we privilege any attempt to gain knowledge over any other? Why, for example, is a spiritual experience a "better" way to learn truth than a scientific experiment, or even using a divining rod?
I think on the one hand, that if we start from a position of belief in God - a God that is external to our reality, then we can suggest that at least from God's end, real communication can occur. That is, God can communicate His will to an intended recipient (even if that recipient filters that communication). Because God is external, He can exist outside of language. And so in theory, communication with God can also be external (outside of language) - at least one directionally. I have often wondered if this wasn't the situation in events like that described in 3 Nephi 28:13-14 -
13 And behold, the heavens were opened, and they were caught up into heaven, and saw and heard unspeakable things. 14 And it was forbidden them that they should utter; neither was it given unto them power that they could utter the things which they saw and heard;
Too often, LDS will compare this to the mysteries of say, the Temple, where we covenant not to disclose certain things. But this really seems to me to suggest that their language was simply incapable of expressing what they experienced. And it isn't just communication - it is an experience that they have, that cannot be spoken.
Having said that, we don't normally get God communicating to us in such an external way (and notice that they were "caught up into heaven" - they are taken out of the world to see something external to it). I am slowly working on an essay for publication titled "Nephi: A Postmodern Reader" (the double-entendre in the title is intentional). When God wants to reveal something spectacular to Lehi, he gives him a book and tells him to read! This is quite distant in terms of communication from the shared vision of the Tree of Life which Lehi and Nephi see (and which is distinctly different for them both - because it is experiential revelation - and they both experience it differently - or at least this is what Nephi tells us).
I think in getting back to your question, we have several issues. One is that we certainly shouldn't see one way as privileged over the other ways. But, and this is part of the key, that doesn't mean that we (individually) are necessarily going to find all paths or directions to truth equal in our own experience. And so what this means is that while we may find one way to be of far greater value to ourselves, we can't of necessity claim that ours is the best way in any absolute sense, or the only way, or even the most appropriate way for anyone but ourselves. Nor can we suggest that in our way that we have found absolute truth. And this is talking about truth. Some truths will be easier to learn in a scientific fashion, or by experiment and observation. Other truths may not be easier (for us as individuals). I always found mathematical discovery to be the closest in my experience to spiritual enlightenment in a secular field of study. Math for me is very visual and experiential.
Next question.
If the subject is merely the product or projection of language and ideology, how can it know anything, much less believe itself capable of knowing anything?
I think when we get to this point, the hypothetical stops working within the practical of reality. There is that old joke, right? What does an agnostic, dyslexic, insomniac do at night? He lies in bed, awake, wondering if there really is a dog. One of the issues here is that these kinds of questions run counter to our experience. We generally don't listen to someone talking to us and start asking all these questions about meaning.We don't tell our kids that they aren't really learning things, that they are merely learning language. And we don't let our curiosity about whether or not gravity will keep working today lead us out a 15th story window to fall down to the pavement below. We aren't so much worried about the absolute "truth" implied in your question, because in many ways, that absolute truth doesn't keep us from going about our business. If we can accept that we are in some ways a product of language and ideology, that doesn't prevent us from deciding that these are still good enough to provide a basis for rational thought and belief.
Next.
If every assertion undermines and deconstructs itself, wouldn't the statement "I know the church is true" be the ultimate acknowledgment that one knows nothing?
This has been for years a rather pet peeve of mine. After all, the church is a noun. It's like saying "that tree is true" or "rock is true". Which, perhaps in some existential way is quite accurate (but ultimately meaningless). Usually, when people say "I know the church is true", they are intending to convey by that statement a list of propositions, which, they believe (perhaps to the point as I note above that can claim knowledge in a rational way) to be true. They don't provide the list of propositions. And most of the congregation can nod their heads, because when they hear "the church is true" they too hold a set of propositions involving the church, which they also believe (likely in some rational fashion) to be true. And in reality, no one in the congregation likely shares the exact same set of propositions. And they are all saying "Amen" to their own version of the church (determined as we note in this discussion by language and ideology and a whole slew of other factors). And we can all be happy that we have, in our many disparate identities as members of the church, found unity.
The other thing is that there are very, very few post structuralists and postmodernists in the church. There are probably more Girardians. So, much of your discussion here wouldn't make a lot of sense to the vast majority of members of the church. But in following your first question, the postmodernist is not going to deny them their narratives and metanarratives. They will simply express a desire that these various narratives (including postmodernism itself) should not be privileged or set up as a metanarrative - as the only way to truth.
So, in answer to your question, nearly everyone who uses the phrase "I know the church is true" will tell you that it isn't an assertion that they know nothing. They do mean something by it. And if we ask them, they will provide us (generally - at least in my experience) with a list of propositions which they feel are implied in that statement. And on some level they can indeed know this - because absolute truth is not, in my opinion, a necessary basis for rational thought or belief. And I think that those who look at language in the way being presented here simply won't use the phrase, because to them (as it is to me), the phrase itself is empty of real meaning.
Next.
How do we reconcile the notion that the Spirit "speaketh things as they really are" with the idea that language is the only thing that really is?
I would say that what you are presenting is a clash of paradigms. Because in Jacob we have this interesting text right? And we read it, and we have to interpret it. But Jacob didn't speak the language he wrote in. He had to interpret it, and to write it down. And we get this interesting notion with Nephi and his vision of the Tree of Life. The appropriate way to understand the vision isn't to ask the oracle (that was Laman and Lemuel's approach) but to ask God to receive the vision. There is something about revelation from God - revelation from something external to our experience - that is of value to us because it is external. And we cannot get that something from a text. We cannot get it from another witness (because, after all, then we don't have this externality to it). Whatever they experienced can only be related to us through language - through difference. If God speaks to us, it is in a personal way to a particular person at a particular place and time. We cannot write it down and keep that meaning. We cannot remember it and keep that meaning. It is gone. But, we can go back for more. I think that to answer your question, the Spirit is a powerful witness because it isn't language. And it can change people - it can take them out of their place and put them someplace else.
Members of the church have always struggled with this. There is always this attempt to ground language in meaning. Joseph Smith struggled with it terribly. He wrote in a letter in 1832: "Oh Lord God; deliver us in thine own due time from the little narrow prison, almost as it were, total darkness of paper, pen and ink; and a crooked, broken, scattered and imperfect language." For him, at times, the solution was the mythical language of Adam - a language without imperfection, ground externally in its meaning. But, of course, that's an entirely different and engaging topic. But I do wonder if the language of the Book of Mormon (which contains a lot of interesting things about language - perhaps as out of place in the 1830s as it would have been 1200 years earlier) didn't influence Joseph's thinking about language.
Ben M.