It would be nice if you could select multiple cell faces and have it plot the combined flow across it. In other 2D models programs, you draw a flow line and they will calculate the flow across that line.
It is a bit awkward, but you can create a 2D connection on top of the cell faces, then for the weir station/elevation you can copy in the underlying terrain. This will you to see the total flow without affecting the computations.
You can create a profile line which will allow you to see velocity, WSEL and Depth across the AOI. Just go to RAS Mapper then on the bottom left click on the Profile Lines tab. Then select the Plus sign box and draw in a line where you would like to see the 2D model output. To view this click on the name of the line you just created and then right click it in the RAS Mapper screen. This will allow you to get the data in table or graphical format.