You need that site param/definer so you can select bindings from the remote source/Project. Without it Bindings will not be available under the Data selector. Sat there for hours figuring this out as it was not added to new Projects by default. I did raise a Bug report but no idea if it was resolved or not?
I found the request (for remote form actions which may well be causing your hassles) but the Bug report is AWOL..