As for the multiple passes, that's actually how it works, the first pass sorts into the 7 most common categories and then quickly works down from there. But the 'runoff' bin is by far the largest at the end of every run. After 3 passes the bulk has completely disappeared and most of the remains are sufficiently rare to be dumped into the right container individually. It goes pretty quickly: first pass: 7 bins, second pass: 7 bins again, but 49 if you put the first seven through again (but in larger volume accumulated from several runs on smaller lots). And so on. Since the bulk of the lego is in those first 49 and the value is in the remainder it doesn't take long to add value to a pile.
As for the mechanics, any obstruction, no matter how well intended will immediately become a point for a bridge to anchor to.