I have one specific question and one general one, but first, my back story. I am a blender novice and have watched a lot of Lydna vids and youtube vids and decided to cut my teeth by making a robot from a Studio Ghibli movie. I thought I got the model looking pretty good, but when I attempted to 3d print it, I had some strange issues. Parts of my model were not printed. The structure underneath was, but the surface was not. I assumed I had made some rookie mistakes, so I decided to go and try and remake portions of my mesh. This time I wanted to look at it in meshmixer to ensure everything was alright. Unfortunately, when I loaded it in, it looked even worse than what was printed out! It looks fine in Cura, but not Meshmixer... the oddness abounds.
I assume I am doing a few things wrong with my topology but I am not entirely sure what. So, the specific question: Is there a way to boolean in a shape without creating a bunch of ngons? I want to cut a hole in the visor as seen in figure 1 and what I have is in figure 2. Figure 1 Figure 2
More general question: Could anyone critique my model (keeping in mind I want to use it for 3d printing, but any advice is welcome). I've attached a link:Google Drive Download, ask if you'd like it emailed instead