1. Reader Extensions has its own interface
http://localhost:8080/ReaderExtensions2. What do you want to test for Dig.Sig?
3. You can use an xml file to merge with you template using FormsIVS. Just pretend the xml came from whatever system. If you want to interact with the external system, then you'll have to install Workbench.
In the end I think it'll be worthwhile to install Workbench. I know it's annoying to uninstall/reinstall but in the end if you're planning to use LC, that might be the right thing to do.
Jasmin