Help me fix my cartridge

This was my first cartridge installation experience and it went very bad. I was installing my new Dynavector 20x on to the tonearm of my Pro-Ject 9.1 by screwing the screws into the threaded holes when I noticed that they were getting tight. I tried the other hole and the same thing occurred, oh oh I thought, better unscrew these screws, they're slightly too big. Then things started to really go bad. The screws inexplicably became very tight as I unscrewed them. I had no choice but to apply more force since the screws were only halfway in and the cart was dangling from the tonearm. Unfortunately the aluminum screw wasn't designed to take that much torque so it broke off in the hole! Darn! I tried to gingerly remove the other one but snap, the other screw broke off also. Man, now I'm screwed I thought. (no pun intended) I have two broken screws stuck in my cart with no way to remove them. I thought I didn't have anything to lose at this point so I tried to drill through them with a tiny drill bit. The first one worked, great, now to do the second one. With great precision I drilled clean through the center of the screw, but now the outer edge of the screw was attached to the threads. So now I can't screw in any screws. Do you have any solutions to removing the remnant of the screw? Can I tap the hole somehow? Right now the cart is attached to the tonearm with one screw but I'm sure two are ideal for the best sound.