Thunderbolt Setup Advice ?!

The Thunderbolt interface has never had it's own unique connector - it's used other connectors. For Thunderbolt 2, which the 2015 MBP has, it uses a Mini DisplayPort connector. The 2015 MBP Mini DisplayPort connector can pass DisplayPort or Thunderbolt 2 signals. Not all devices with a Mini DisplayPort connector will do that. In looking at the Dell UP2516D monitor, it does not mention Thunderbolt that I can see so it's has a Mini DisplayPort connector which will receive only a DisplayPort signal. In other words, for what you want to do with this monitor, Thunderbolt isn't used.

In order to pass the USB data, you need a separate cable to connect the USB port on the computer to the USB upstream port on the monitor. There may be some limitations on the USB port use - you'd need to research to be sure.

If you don't want to connect multiple cables from the MBP to the monitor, then you will need to get a true Thunderbolt monitor which carries the USB signal in the Thunderbolt port. That will cost more than non-Thunderbolt monitors. I looked at one article on the web listing alternatives to the Apple Thunderbolt monitor and the two Dell monitors it listed didn't have Thunderbolt input. I looked at one result in a search for "monitors with thunderbolt port" and it came up with the LG 34UM94-P monitor for $849 but it looks like there's a separate USB port. The now-discontinued Apple Thunderbolt Display had a single Thunderbolt cable and multiple USB 2 output ports. So if you want the single cable to the monitor, you'd have to do some research and plan to spend more money.

Unless you really want just the one cable, I would stick with a DisplayPort monitor and when buying a cable, get a cable that carries the DisplayPort signal - you don't need one that can carry a Thunderbolt signal.

EDIT: You can get a Thunderbolt 2 docking station (it has to be 2 NOT 3). They are about $200. An article I saw indicates that the USB 3 performance (important if you are going to connect an external SSD) takes a hit when using these docks. http://thewirecutter.com/reviews/best-thunderbolt-dock/

 

From what I am reading in the manual you need to connect a USB cable from your computer to the upstream port in the monitor to use the down stream ports in the side and back.