I don't see any way they can stop the trial until they've confirmed that 250 events have take place via adjudication.  What are they gonna do....go until 300 events and then "hope" that of that 300 that 250 are confirmed as the narrow 3 point MACE detailed in the trial protocol.  

Imagine a scenario where they halted the trial and then said "whoops, our bad...we thought it was 250 but some of the events we thought were MACE were other things, like brain tumors that caused us to think stroke".  

I can't imagine that happening...