On high volume stocks, this is not a major issue, but if DE is on the inside in thin names, the difference in price can be substantial. I dont see how any displayed quote provider should be unexecutable, or why they would allow it. I don't believe DE fees are problematic, probably the opposite.
Wondering if NYSE would not ship an order to DE if that was the best price. Even if so, IB would have a problem with its stops, and spread orders , since they would not be 'seeing' the best price.
For NMS stocks, all market centers will be required to ship the order out to the best inside quote. Unless the broker marks the order not to be shipped, but then the broker would have been required to have checked all market centers to avoid a trade-through of a protected quote. I suspect IB will take the path of expanding SMART routing to make sure all NMS market centers are covered vs. handing it off as this would allow for faster routing, better control, etc. -- all IB differentiators. And this would support breaking orders across markets, spreads, etc.... whereas handing it off would not. And hopefully, non-NMS stocks will benefit from the infrastructure as well. Just my speculation.
i noticed directedge recently on stocks which didn't have it before. can't confirm if it's on *all* ,howevre.