In order to protect people from odd lot executions REDI matches odd lots only with other odd lots and never outside its book. That's why you'll often see locked and crossed markets on REDI's own order book. As for locked markets with other ECNs after hours, that happens when an existing order on REDI's book is later joined by an order on another passive ECN on the opposite side of the market. REDI will only invoke its proactive mode if it determines that the order is marketable at time of submission.
I don't know about the ah thing cause I often see 500+ share redi and island orders lock each other. When i traded with terranova, you could see the arca overhang. I actually could pull an island order if i was fast enough, but inca orders usually get 3-5 seconds extra.