Dell U2715H has a USB-B 3.0 upstream connector for its internal USB hub. If I connect the upstream port with a USB-B to USB-C cable to a 13" 2017 MBP will it charge the laptop? Another way to put this question is: can I use the 13" 2017 MBP in closed-clamshell mode with the Dell U2715H connected through a DP to USB-C and a USB-B to USB-C cable?
The monitor will not be able to charge the laptop, no. Therefore the MBP will not by default work in closed-camshell mode when connected only to the monitor and not to another external power source.