hi, on your third point, when Legacy custom forms were created, these were shared system wide by default. When I transitioned to the new custom form builder, I noted that these were not shared system wide by default, so I submitted a bug report for this, to have them change it back to achieve parity with Legacy.
We're accustomed to folks being able to immediately see and fill out any custom form we create and the majority of our forms need to be filled out by everyone in the instance. It's rare indeed that we have a custom form that we need to select an audience to fill out.
see also:
https://experienceleague.adobe.com/en/docs/workfront/using/administration-and-setup/customize/custom...
"By default, when you create a new custom form and someone attaches it to an object, any user assigned to the object can view and fill out the form. This includes users with Request licenses and external users." (this is what system wide access is supposed to allow)