This is a question I initially had too. After some drawing and observing, I think the only true verticals exist when the (1) the edge is facing the viewer directly or close to it, or (2) the object is on the horizon level or in 2 point perspective. As you start looking down or up on something, lines begin to converge more and more - sometimes subtly if the object is further and sometimes very radically with lots of distortion if the object is close to the viewer. Because when it’s close to the viewer the VPs move closer together so you get lots of distortion. I think it was d’amelio’s perspective book which basically said some simplify to complete verticals but stuff looks weird when you do that, and I tend to agree (which is why I went down this rabbit hole myself trying the blob to boxes exercise, something felt off and it was that doing true verticals was ruining the angles of the boxes which are 90 degrees but of course in perspective.) In the screencap above you can see the lines are subtly converging and not true verticals. You can test this out for yourself with the zolly app which is really helpful with confusions to remind how things change when it’s really subtle. As for tilting it, it’s up to you how much you’re comfortable with!

We'll cover it with three simple letters in a coming lesson. Maybe even a song...

My understanding as a fellow student is the line across in the three-quarters view model indicates the front plane of the face. I think it starts at the place where the eyebrow curves, showing the portion of the face that is the most front-facing towards us, the viewer.