@unaccomplished
Hi,
just tried your config in my setup and everything worked as expected.
The question is…where are the differences in your and mine setup.
As a first try you can replace embedElementType: "webview," with embedElementType: "iframe,".
If the embeded pages does not block embedding iframe should work and you do not weaken any security settings of the electron browser.
Additionally you can replace "frameborder=0", with "frameborder=1",. You should see two boxes at least then. Even if the sites could not be embedded.
@sdetweil Are you sure you set the security settings of electron? Without them electron will not support webview and will not display anything.