Forgive my short reply–I will aim for “lucid brevity” (to borrow from John Calvin!).
1. On the silly ham salad analogy–please do not read too much into it. As you know such rhetorical devices cannot be pressed too far. The illustration was not intended to say that syncretism is combining all the bad elements of various religions (indeed some of them may be very, very good elements). It was simply a very pedestrian way of saying that when you begin “mixing”–what you have is not what you had anymore. It is something new or different.
2. As to your wine or beer analogy–I do like it. Some have called this the evolution of religious ideas–however, Christian theologians have rejected this on the grounds that Scripture is revelation (God’s words through and to man). Theologians use the expression “progressive revelation”–meaning that over time God revealed more and more of himself to man–imagine a plot developing over time. There is an evident “enlightening process” in the Biblical narrative as God reveals more of himself. (Abraham, to Moses, to Paul–there is development.) There is a certain “drama” in Scripture–with Christ as the hero–the “mystery revealed.” . As such–you certainly do see a “fermentation” process (to use your analogy). The “wine” (story) gets better with time.
3. On the relationship between Christianity and religions (often referred to as Christianity and Pluralism)–I cannot pretend to respond in a paragraph–but let me make a few points. First, Christian theologians in general do not believe that all religions are evil and contain only bad elements. They do believe that “image bearers” are capable of having good, noble, and wise thoughts–and that most religions contain elements that are admirable. Second, Christian theologians (especially missionaries) often use existing religions or philosophical ideas as “bridges” to help people understand the gospel. (Historically, some Christians have been better than others at this. Some have been downright ignorant and cruel unfortunately.) This is not the same as saying to another religion: “Your story is true.” Instead it is a way of saying, “There are things in your story that will help you understand the true story.” Third, for Paul (and Christian theologians in general), Christ is “The Occam’s Razor”–cutting away all the extraneous stuff that one does not need for salvation. In other words–anything that diminishes His person and his work on the cross–must be “cut away.”
4. On the relationship between Christianity and literature–the record in Christianity is mixed. An early theologian named Justin came along in the 2C and argued for a positive relationship between Christianity and secular thought. His famous dictum was “Whatever all people have said well belongs to us as Christians” (our modern version of saying that even a blind squirrel finds the occasional acorn). Tertullian (another theologian) countered, “What does Athens have to do with Jerusalem?” He spoke for some when he decried the use of Greek philosophy in theological debate. Augustine came along in the 4C and spoke for most (and really won the day) when he said: “Pagan learning is not entirely made up of false teaching and superstitions…” He is known for his analogy (and it is only an analogy) of the Israelites taking gold and silver with them out of Egypt during the Exodus. Thus the infamous “plundering the Egyptians” (there is good to be found even among pagans). In short, Christian theologians in general quote other thinkers–sacred and secular as evidence that people are made in God’s image and trying their best to understand who God is. They then point to “special revelation” (scripture) as the source of “authority” (to use your word) in helping us rightly understand this mysterious God. (for a short overview of what Christians think about this–see Alister E. McGrath, “Christian Theology: An Introduction”, p. 16-19 and references).
5. On ancient sapiential literature–and the relationship between Solomon and the “Instruction of Amen-Em-Ope” (as well as some near direct quotations in Proverbs from Amenhotep)–I think this illustrates Augustine’s point (and I’m with him here) that we can “plunder (take from) the Egyptians” for wisdom. Christians believe that people are image bearers and capable of looking at the world and making wise observations about life. Christians can and should interact with the world around them–even using the genres of creative people (the genre of wisdom literature was in vogue during the reign of Solomon) to articulate the Christian message. I believe this illustrates beautifully the fact that God nods in affirmation when unbelievers say wise things (we might say “God, through Solomon, quotes Egyptian sages”–interesting thought!!!). And he hopes, through wise instruction, to lead them to the Source of that wisdom. (Again, see Justin’s quotation above.)