Adding the possibility to modify an existing data provider is a part of the backlog of the Views service but it hasn't been scheduled for a certain release yet.
At the moment, you can create a new data provider, make the needed changes and then go to the view in question and change the data provider that it uses. The old data provider can be deleted when no views are using it.
As for the second issue you reported, not being able to select a field from the catalog, we can't reproduce this behavior using a Progress Data Provider. Can you confirm that you went through the Catalog File step
when adding the Progress Data Provider?
To confirm that all is ok with the catalog, you can go to Code and find the .app.json
file in the Project Navigator, there locate the catalogContent key and see if all types are listed there.
If the problem persists, you can open a support ticket and send us your project, so we can test it locally and see why the dropdown doesn't appear.