Re 1), I like Mike's talk from the 10th anniversary of D3. It came while working on Plot but before it was officially released; a few of the points relate to the high/low-level thing and anticipate Plot:
Video: https://www.youtube.com/watch?v=Sqeymt1mqAw Essay: https://observablehq.com/@mbostock/10-years-of-open-source-v...
Re 2), Plot returns a regular DOM node, so you can select any part of it and do anything you’d otherwise do with D3. Plots also expose their scales: assuming the DOM node of a generated Plot is named myPlot, you can call myPlot.scale("x"), and then feed that domain and range into a D3 scale. I can find some examples later if you’re interested.