I have got my Mac Mini to display the full 3440x1440 resolution on the U3415W by connecting the mDP end to the Thunderbolt port and the DP end to the DP in port. I did however need to turn DP 1.2 off in the onscreen settings. I have a PC connected using the mDP port of the monitor and can switch between them using the onscreen menu.
With DP 1.2 set off it is not possible to Daisy Chain another monitor. I also setup picture-in-picture with the output from the Thunderbolt Port as the main display and then had the HDMI output also from the same Mac Mini in the PIP window. I had mirrored display set off and so I had the weird display of two desktops from the same Mac Mini 😉
I hope this gives you some helpful information.