I agree with the previous answers; in particular, considering each question:
1) What happens if the interviewer is not able to cover all the questions within the allotted time?
At McKinsey the interviewer will usually cut the question and move to the next area, so far that you are not able to complete your analysis in the allocated time slot.
2) Will the interviewer just stop after the time limit?
As mentioned by Steven, there is a bit of overrun possible, but normally no more than 5-10min
3) Also, is this bad as a candidate?
Of course the best thing would be to complete each session in the correct way. However, if you do everything well and just go longer in one area, you can still move to the next round, as mentioned by Wouter
4) Is doing the case accurately better than doing it quickly?
There is a trade-off between the two extremes. Neither perfectly and slow nor quickly inaccurate are ideal. Having to choose between the two, slow and accurate is usually better, in particular for math. But, as Steven mentioned, being too slow could affect your performance as well.