So, I determined that my "issue" listed above was due to a defective mDP>DP cable - I notified Dell and they overnighted a replacement to me. So, now I've got my two Macs - a 2012 i7 Mini Server and a late-2013 rMBP connected to both monitors, each via the DP and HDMI ports. It's just a matter of using the monitor control panel to switch interfaces.
I did test one "feature" with my rMBP, daisychaining both monitors - rMBP DP to P2715Q in via mDP>DP cable>P2715Q out to P2715Q in via mDP>DP cable. With lower resolutions, I was surprised to see the "mirrored" monitors showing the same screen at 60Hz - it's a shame I can't get both monitors working on different channels @60Hz, despite have integrated and discrete graphics on my rMBP over mDP/DP.
I'm envying larrysing1963 for owning one TB and one DP monitor and getting 60Hz on both monitors. I'm grumpy that none of these monitor makers support Macs (pretty much at all...).