CQS/UQDF is standard as a failover feed for HFT firms, at least ones I've worked at.
You're right about how Reg NMS works for IOC's. For DAY orders, you must clear all shares at all prices equal to or better than X (in your case, you must be all shares on EDGA/EDGX at both 9.99, 9.98, AND 10.00).
"Over our dissent, the majority voted to adopt Regulation NMS on April 6, 2005, approving a trade-through rule
protecting quotations at the ‘‘top of book.’’"
You're right about how Reg NMS works for IOC's. For DAY orders, you must clear all shares at all prices equal to or better than X (in your case, you must be all shares on EDGA/EDGX at both 9.99, 9.98, AND 10.00).