Well the problem has been fixed... Kind of.
I was playing around with the lenses off camera and saw that both worked perfectly when fired manually, no trouble at all. So I'd cock the shutter on the lens, mount it to the camera, fire the shutter, and it works fine. But just that once. When the shutter was cocked again by turning the film advance knob, the mirror would slap up and everything seemed to be working, but the shutter simply stayed closed. I couldn't figure it out, I was thinking I was going to have to take the lens off and manually cock the shutter for each individual frame!
So I talk to the guy that sold me the camera and he says something strange about the mounting of the lens. Apparently, you cock the shutter on the lens (as usual before mounting it), line it up on the body, and twist the lens itself as you twist the locking collar (clockwise if you're looking into the lens). I mess around with this technique and finally have the shutter opening each time! The weird thing is, when mounting the 85mm lens, you have twist the lens the same direction as the locking collar, and with the 55mm, you have to twist the lens the opposite direction to get the shutter to actually open every time.
So now that I have the shutter firing consistently, I went out to shoot and sure enough, it seems to be functional. No blank frames.
Since I still had no idea what was causing the extreme overexposure of the first three frames on each roll, I taped up the back of the camera before shooting, and those frames did indeed came out normally this time. I have no idea why the issue would just be with the first three frames though? Especially since I've shot each roll at night, it didn't make sense that they were light leaks...
Now that it's working (not really like it's supposed to be, but still...), I'm not sure what to do with it. Just deal with these strange quirks and work around them, or actually get the camera working like it should be.