Thanks for the thorough critique!

Regarding the back vertical edge on my plotted perspectives not being perfectly vertical, I thought this was because I was supposed to connect the corners, or Xs, where the converging lines meet. I thought this was more important than having the back edge be vertical, so I was focusing my efforts on having those back corners try to line up on top of each other, but was confused, and still am, on how to improve this. I re-read the instructions and I'm inferring that I should have done it this way. It felt odd to know that I'm drawing them slanted but I guess a vertical line is still probably better then an accurate corner, visually speaking.