In terms of the pen usage, while it definitely looked like the clean-up pass was done with a fineliner, there are definitely situations where the results can be hard to distinguish. That said, the bigger issue was less the switching of the pen, and more the clean-up pass itself. When we change pens, it pretty much requires us to trace back over the entirety of the drawing because of how when mixed, fineliners and ballpoint pens tend to stand out, so it'd look strange to just touch up a few areas with what would be a much thicker pen.

We advise students against clean-up passes because they tend to put us in a "tracing" mode - meaning, we're focusing on following the lines on the flat page, rather than remembering that our strokes are meant to exist in three dimensions, and be drawn with confidence. While I didn't strictly notice that issue in your work, it's still something I wanted to bring to your attention both as something to avoid when applying the exercises from this course, and in terms of making sure you keep a close eye on the instructions.

As to the orthographic plans, by landmarks I basically mean any sort of corner or feature that you would need to make a decision on at that phase. I marked out a few here - each of those, working from that orthographic plan, would still have to be guessed at or estimated when moving to 3D space. As explained here, those are the things you already want to have decided upon before moving to the 3D construction - or in the situation where you missed some, it's probably best to go back to the orthographic plan, make the decision there, then continue (instead of trying to eyeball it).