For the nut on the clutch, have you tried having a 2nd person hold the clutch bell still while you use the ratchet or impact wrench? A strap wrench works well for this. Also, a longer piece of pipe put down over the handle of the ratchet will give you more leverage to work with. These nuts are tight for a reason: you don't want them coming loose and falling off!
In the 2nd picture, that screw is the bowl drain screw. It will allow the gas in the float bowl at the bottom of the carburetor to drain out through the tube that is right there. If you open it with the fuel lines hooked up and gas flow to the carb turned on, you're going to be leaking out the fuel from your whole tank onto the ground. Keep that screw in the closed position unless you need to open it!
