Is it ok to "over tighten" laptop heat sink screws? Is it ok for one screw being slightly less tight than the others?

Overtightening a heatsink screw could strip the screw, crack the cpu, or damage the socket.

Uneven pressure on the cpu may cause an uneven gap between the cpu and the heat sink, causing one side of the cpu to overheat.

Of course, just because one screw can't be tightened all the way doesn't mean that the heatsink isn't properly seated. It might be fine. That is in part why there are springs on these things.

There is a range of tightness that will work. Overtightening other screws to make up for the stripped one is probably a bad idea, as it is less likely to seat the heat sink evenly.