Thanks boatman. Separating the issues:

Broker selection: Yes, conflict of interest is a significant negative. Of course, this pre-supposes that none of the NDD brokers are colluding with the liquidity providers...

Education: There don't seem to be that many "levers" a broker has (given that costs are specified) - perhaps only price & spread & tick frequency & timing? - so I was wondering if there are any documented examples of how a DD broker misuses these levers in comparison to what comes from an NDD broker(s)? Perhaps someone else has examples...

Thanks.