OK… I see one of my problems. Look at the attached images.
In the above image, I did as you suggested and copied my stencils from Dropbox.
Then, I navigated back to locations and then into Omni Sync Server, as seen below:
The stencils were gone! What! You see that little tab bar at the top? That is not visible, by default, until you SWIPE down (see the first image).
When I tapped on Show Stencils, I see this:
My stencils were there ALL THE TIME, I just never saw them because the filters were pushed above the top edge on my 12" iPad Pro.
Your UI team needs to revisit this interaction model :-)
I am still working on my second issue, which is: if I open a diagram to edit, how do I open a stencil and drag/drop an item into my drawing?