4K 60Hz over DP with Late 2013 Macbook Pro 15" + Dell P2415Q?
Okay so I got 60hz working at 4K. Despite the fact that I swear I already tried this, the key was turning off MST.

Unfortunately, I can't recall why I turned MST on in the first place. Did I create my own problem? I will say that I was unfamiliar with what Multi-Stream Transport was when I read the Apple support doc that I linked to in my question, and the doc was written in such a way to suggest that it was necessary for me to have on to get 60Hz on my MacBook Pro model.