Based on what you've reported I don't see how a DP monitor could work. DP requires link training to bring up a link so there is no way to manually force a mode; both sides (monitor and card) have to negotiate the link. If the aux pins are not working you cannot get the dpcp or check the link status or get the DP EDID.
I suspect in the fglrx case, it too sees the sink as CONNECTOR_OBJECT_ID_DISPLAYPORT. It may use that alone to determine connected state rather than also checking the dpcd. However, it also seems unable to get an EDID which is why it defaults to 640x480. If it tries to light up a DP screen without working aux pins, there's no way to get the dpcd and train the link so it won't work.