While I disagree with a majority of your post the one I want to bring up is the circumstantial modifiers bit - because I was confused that you weren't seeming to notice it - as well as the idea of multiple choices in dialogue. To the former: I immediately noticed circumstantial modifiers, I was rolling as a tiefling warlock with my proficiency in persuasion, I was noticing dialogue options that were focused on persuading a person toward something they were already leaning toward had a much lower target number (generally a 1, though in some cases a 5) for my to succeed while trying to persuade someone against their views (specifically Kagha) required a 15 or better on the roll to make happen. I also found, when replaying it, that sometimes Deception or Intimidation had the lower rolls made available to them vs Persuasion in similar encounters - the amount of times I had a 10 roll was minimal compared to the 1s and 5s - at least when it came to my Charisma checks, my character was a himbo until he received a crown of knowledge.

Now for the multiple choices in dialogue - you used Nettie as an example. Nettie is a poor example for an encounter, to be honest. I much prefer using the first time you encounter Astarion as a perfect example of how the game decides the outcomes - considering the choices you have available to you during your meeting with Astarion being physical (a head butt), dialogue, or just to wait and hope everything goes right. Kagha is also a great example, as you can make use of your race (in my case tiefling) to try and push for a better outcome. Both Kagha and Astarion are, what I as a DM, would classify as an actual encounter where Nettie is a Plot Vehicle. She isn't really meant for anything more than to be there as a stopgap on the Plot in showing just how the world views the affliction. There is nothing wrong with the Nettie encounter from a campaign perspective, though - it's perfect for what it's meant to be. It isn't, however, the measuring stick you should be using for determining how dialogue encounters go when you're using the absolute worst example of one.