The hotshoe may be an easy fix, two of mine had the same problem.
You'll need a set of jeweller's screwdrivers. First use the smallest flathead driver to gently lift the sprung inlay up at the front and slide it out backwards. This will reveal four crosshead screws. Undo these and the hotshoe will lift off (the MX doesn't have any internal nuts to drop into the camera AFAIK - the screws go into tapped holes in the top plate).
Underneath you'll find a brass tongue which is supposed to touch the contacts extending down from the hotshoe. Bend it up slightly and check the contacts now touch. Reassemble (making sure that the sprung inlay goes back properly - it needs to be held firmly against the hotshoe as you slide it back in to make sure that it hooks over at the back), fit a flash and try it.
The Cosina lenses may or may not be good, only advice I can give is to try them. Some 3rd party primes were pretty decent lenses.
Not sure which lever you mean - if it's the one on the front of the camera then it's the self timer and depth of field preview. Push it toward the lens mount for DOF preview, for self timer pull it away from the mount. To trigger the timer press the little circular button at the 12 o'clock position, providing the camera is wound it will fire when the timer runs out.
There is also a locking collar around the shutter button to stop you from accidentally firing it, although I tend not to leave mine wound on as it's hard on the shutter springs if you leave it that way for weeks or months at a time.
Metering - you have LEDs on the right hand side of the viewfinder, to turn it on half press the shutter button. If you pop the winder lever out (rather than having it stowed against the body) then half press the shutter it will click at the halfway point and keep the meter turned on. Push the winder lever back in to switch it off. A green LED means correct exposure, yellow means over or under by a stop, red means over or under by more than a stop.
The meter uses two button cells in a compartment in the baseplate, just like the K1000. The other similar fitting is for a motor wind. Use a 2p coin to unscrew them, not a screwdriver (wish the last owner of my K2 knew that, I wouldn't have had to replace the battery cover then!)