To add to your own answer....
If "it is not in heaven" applied to wisdom in general, why the other 19 books of Tanakh? I wouldn't call them situational, because there were schools of prophets in each generation through to the end of the First Temple period. (And maybe slightly beyond; prophecy doesn't end until early Second Temple.) The prophets and prophecies we know about are the ones that have messages deemed valuable for the generations.
"It is not in heaven" means that G-d gave us the halachic system and wants us to use it to redeem ourselves. Not to expect those "general mitzvot" to be defined by Him, but to work with what He gave us. After all, what is greater living up to the "image" of the Creator than using our own creativity rather than relying on His?
In my book (Widen Your Tent, sec. 2.2) I compare this with Hillel's summary of the whole Torah to meet a candidate for conversation who insisted on being told the whole thing "while on one foot". Hillel said, "That which you loathe, do not do to another. That is the whole Torah. Now go and study." Hillel describes the whole Torah as being an implementation of a very natural concept of morality -- reciprocity and empathy. Just as the Torah is founded on natural morality, I argued that it can only be interpreted by someone who experiences natural morality. As it says shortly after your quote (v. 14),
"Rather, the matter is very close to you, in your mouth and
heart, to perform it.”
Related is that the objective Heavenly Truth of Torah admits multiple valid answers. We have to find what they are, and of those choices, which is right for us. (For more on this tangent, see "How does “eilu v'eilu” work out with an absolute truth?" I personally like the "two shadows of a cylinder" metaphor, which seems to illustrate the Maharal's position in particular.)
The Torah tells us how to best implement natural morality, and how to hone ourselves to be better at it. And therefore, its interpretation cannot be separated from the history of the Jewish People.