Unknown about USB switching. The DDM command line usage is found in the readme.txt =
C:\Program Files (x86)\Dell\Dell Display Manager\readme.txt

The UP2516D DDM User's Guide does not mention USB so it must be through video input and DDC/CI Enabled in the UP2516D Menu.

Can't be sure this will work since it depends on the FW, but try "ddm.exe /SetControl E7 xxyy", where xx is the input you want linked to USB upstream port #2, and yy is the one you want linked to USB #1, and DP=0F, mDP=10, HDMI1=11, and HDMI2=12.

So to link DP with USB #1 and mDP with USB #2, xxyy would be 100F.

Does that make sense and work?

Yea, that made sense but it did not respond.  I downloaded softMCCS and GetSupportedControls lists  02,04,05,08,10,12,14,16,18,1A,52,60,AA,AC,AE,B2,B6,C6,C8,C9,D6,DC,DF,E0,E1,E2,E4,E5,F0,F1,F2,FD.  So E7 is not listed.  So have to assume that one is not it.  Codes E0 to FD are listed as Manufactor specific. Do you know what these codes are for?   Would you share.  Thanks....

I was able to get /SetControl 60 <0F,10,11,12> to work.  So /SetControl does work with ddm.exe Version 1924.  Code 60 is input source select.   Be Well..

I'm afraid your monitor FW must support software control over USB upstream port assignments. If it does, you'll find an "Input Manager" tab/page in DDM; if it doesn't, you won't.

I have a U4919DW, I have the same problem and I'm stuck at the same place. Impossible so far to figure out the value to send via DDC to the e7 record. It is really a pity that Dell does not publish the DDC spec! Considering that they do not provide portable solutions, that is really annoying.

Try: ddm.exe /SetControl E7 FF00