Something written can often times be more misinterpreted than something said, but the situations are different.
In a case of he said she said, you are arguing over what was said or heard and you don't have the source to verify, except you do because you can call that person and ask.
With a written document, you have the source, and no one disputes what's been said, but often what was meant or how it was said, especially when dealing with a document that didn't come with written punctuation, and is constantly being looked at by people with thousands of years of separation.
So for example, is this the correct sentence?
“Most of the time travelers worry about their luggage”
Or is this the correct sentence?
“Most of the time, travelers worry about their luggage.”
And since we can't call up the author of the Torah to ask questions, the text is left ambiguous in that regard. Another example is we have forgotten what certain words mean, and we therefore have many different interpretations of the same word, because its original meaning has been lost.
There are also the cases of euphemisms. So let's just say for the sake of argument that you become a prophet of our time, and you write a book, and you write the sentence "It was raining cats and dogs." Everyone in your generation, who speaks English, would know that you are using a euphemism to describe that it was raining very heavily. But people 4,000 years from now who don't speak English will run the risk of thinking that there was a miracle, and that God was making cats and dogs fall from the sky.
The next level of potential misinterpretation grow further when you start to deal with complex and usually changing ideals of morality. In the time of the Torah, one of the great goods you could do in that world was to marry your sister in law once your brother, her husband, had died. That is hardly considered a good thing anymore, because women are able to choose a new husband for themselves in this era, and we don't have the issue of ancestral land anymore. If someone were to try to do this law of Yibum (levirate marriage) now, even though it's clearly a miswah d'oraita, there would probably be a communal uproar. In this vein, if one were to try and write all the nuances of ethics of the time, then you would end up with divine ethics of a particular time that would be unchanging, which isn't helpful in the long run. Therefore it makes more sense to have sets of rules, and to have sages throughout time write the mussar, because they will always (hopefully) be updated with the world in which you find yourself.
As for the derech eres point. Your point is valid, but i would suggest listening to some Rabbi Fohrman over at alephbeta.org as he has some interesting thoughts on that. Including the idea that the Torah does indeed incorporate some laws that are Derech Eres, but he also points out that the Torah might also be overturning certain Derech Eres. One of his main points is this law in Devarim Chapter 21
טו כִּי-תִהְיֶיןָ לְאִישׁ שְׁתֵּי נָשִׁים, הָאַחַת אֲהוּבָה וְהָאַחַת שְׂנוּאָה, וְיָלְדוּ-לוֹ בָנִים, הָאֲהוּבָה וְהַשְּׂנוּאָה; וְהָיָה הַבֵּן הַבְּכֹר, לַשְּׂנִיאָה.
15. If a man have two wives, the one beloved, and the other hated, and they have borne him children, both the beloved and the hated; and if the first-born son be hers that was hated;
טז וְהָיָה, בְּיוֹם הַנְחִילוֹ אֶת-בָּנָיו, אֵת אֲשֶׁר-יִהְיֶה, לוֹ--לֹא יוּכַל, לְבַכֵּר אֶת-בֶּן-הָאֲהוּבָה, עַל-פְּנֵי בֶן-הַשְּׂנוּאָה, הַבְּכֹר.
16. then it shall be, in the day that he causeth his sons to inherit that which he hath, that he may not make the son of the beloved the first-born before the son of the hated, who is the first-born;
He posits that this Law is [possibly] a response to Jacob making Joseph the firstborn and thereby causing all the sibling rivalry.