Binding on whom? In general, a DSMB can stop a trial whenever it deems such action appropriate, regardless of any prior agreement with the trial sponsor.
We have no way of knowing that -- that would be an SPA provision, yes? I'd be surprised if it weren't, and the DMC would almost certainly reocmmend stop for futility if the hurdle were crossed -- and the DMC recommended a continue for START.
The pre-final alpha spend here is going to be tiny. Final alpha will be something like .02495 after the two tiny interim spends.