suck on the opposing end while the machine is off? all kidding aside... new pump could be faulty. try to verify that the pump works at all. i would complain to the person who sold you the pump and see what they can do for you. you may have ruined the seals that make it work by not priming it properly before the first run. use the gravity siphon prime method. put the pickup tank above the output and gravity prime it. (i'm imagining a pump with a pickup hose.)

since you already had it apart, how bad would it be to get another keyboard? sorry i'm not a mac user but i had a really good experience replacing a dell keyboard. only 15 bucks shipped and less than 15 minutes and i had a brand new keyboard on a still good laptop.

is the on/off toggle switch broken? i would get a continuity tester and check to make sure that it works properly. it really doesnt sound gear related. can you describe the problem further? does it still work at slow speeds? if it doesn't it could be that the switch is fine and that the motor is on it's way out. any funny smells? does it run differently when first started vs after some heavy use? older kitchenaid mixers are worth a repair job hang in there. try some experiments and get back to me.

sometimes impact can help with overtightened screws. i would have a friend tap (lightly) on the top of the screwdriver while you turn it and while pressing down really hard on it. sometimes heat can help loosen things up. hold a soldering iron to the top of the screw head to heat up the screw. try at your own risk. these methods i've used on cars but never on computers. the dremel idea is one to consider but really risky due to the mess. i've also heard that tightening it further can break it loose. (sounds crazy but it can work.)