Ok:
1 & 2: The photo is a bit high angle to see whether anything is broken, but that switch definitely needs to be clicked by the screw just as the extruder nozzle tip touches the bed. If you need to add something (like one of those brass standoffs from the bits left over from building a computer) to make that happen, even just for the moment, then please do so. I think we could confirm if you share a picture of a nice close front view of that switch and screw with the bed at the top (where it's jamming!)...
3. The alignment between the z thread and the nut will be the worst at the bottom of the travel. If the bed has hit the bottom of the case, it may be skewing the bushings on the rail which could contribute. Also, the lubrication is probably not so fresh down there. It's likely just stuck - it's making the buzzing noise meaning it's trying to move, but evidently having trouble. With the printer off, manually rotate the z thread (clockwise, looking from above) to raise the bed a bit, and then power up and try raising the bed (down arrow in RH remember
) again. You should be able to fix this in future, but save that for when you've got this z limit switch issue sorted!
SD3. Mk2b + glass, heated enclosure, GT2 belts, direct drive y shaft, linear bearings, bowden-feed E3D v5 w/ 0.9° stepper
Smoothieboard via Octoprint on RPi