'94 Sunbird Engine Trouble - STUMPED!

I just bought my friend's '94 Sunbird as a commuter car. It has the 2.0L engine and manual trans. I drove it a whopping 6 days before the engine just died on me while going about 70 mph down the interstate. I trailered it home, pulled the timing belt cover, and discovered that the belt apparently slipped about 4 teeth on the cam gear. The belt looked perfectly fine. No cracking, no teeth missing, nothing. I checked cylinder compression, and there was zero compression on the cylinders. So, I pulled the belt, aligned the timing marks, and put the belt back on. I then had about 150 psig on cylinder pressure (I checked #1 & #2 cylinders). I tried starting it, but it never would fire. I sprayed some starer fluid down the breather tube, but when I turned the engine over, it all blew back out. ??? I then checked each cylinder with compressed air and discovered the #4 cylinder exhaust valve won't seat. It just blows on through. I assume I have a bent valve (although I thought this was a 'zero-clearance' engine where you don't damage valves with a timing belt breakage). I want to fix the car, but I'm worried that after pulling the head and having the valves repaired/reseated, I still haven't fixed the cause of the whole problem. What could have made the belt slip in the first place to damage the exhaust valve? Could it be a water pump seizing up? I'm stumped as to what the actual root of the problem is, and I don't want to dump a bunch of time and money into this if I'm just fixing the symptons rather than the cause. Does anyone have any idea what I could be dealing with? Any help is greatly appreciated. Thanks!