The other 1 M shares will be cancelled back. The idea is that if one gets a fill it will be cheaper than going to nyse/nasdaq but if you don't then just send the residual to the primary exchange. JPM (JPBX ats) and Posit have very similar agency close cross offerings that work the same way although with different cutoffs for sending orders/matching.
It might be. As I understand since the cutoff is 15:35, you will know how much was matched at that time and the remainder you resubmit to the main exchanges. That makes it a no-brainer to use with instant savings.