Sorry for your frustration!
Originally Posted by mdwsta4
It does not matter how hard or how long I press the shutter button (this seems to be the only comment I can find online regarding a fix for this issue).
If it's like the videos it's probably an electrical problem with your camera. It's doubtful that, if the fresnel/mirror seems to "slap" the back of the camera and the shutter opens like in the videos, that not holding the button is the issue here.
A working SX-70 shutter maxes out at ~14 seconds, but I've had a few poor preformers that went as long as 30 seconds. So in good lighting (or pointed directly at a light-source) put the film in the camera, eject the darkslide, press the shutter, and see if it ejects the film after that span of time. If it does, that camera is working but has a degraded electric eye. If not, something is electrically amiss elsewhere.
In any case, try to find one that works or send in the one you have to be repaired. Hope that helps!
EDIT: You can use an empty film pack as well, as long as it has an ok battery - the camera doesn't know any better =).