Again, as I stated, it depends on the monitor and how it is wired. If the monitor uses MST to drive two virtual 1920x2160 monitors, then 60hz is possible. If it uses MST to support daisy-chaining, and thus, must drive the full 3840x2160 on a single one of those lines, the maximum is 50hz.
Your Dell up2414q is one of the former. You'll notice it doesn't have a displayport out for daisy-chaining.
Exactly: 2 virtual with MST as a single 4K with no daisy chaining @ 60 Hz.
About the rest: you're right, of course.