1) This does sound like a bug. I'll look into it for the upcoming release.
2) Because of the special things we do to the form in order to get it to work with the database, particularly on update pages, forms cannot be edited with form builder after generation. Forms should be edited with Dreamweaver directly when possible. When major edits are necessary you would have to re-run the wizard.
This error was reported and will be corrected in the upcoming release. Basically if you try to open form builder with an existing form selected it tries to import the form elements into the interface and that process causes this error. We will have it fixed soon, but you still wouldn't want to edit a form created by Data Bridge in this manner since it would remove some necessary code.
4) Form Builder was always designed as a Form Builder and not a form editor. We added basic re-entrance, but re-entrance really overwrites the form and any edits made by hand. Complete re-entrance is almost impossible given the ability to fully edit the form within dreamweaver after it has run. You expect it to keep validation messages... what about inline css added? style changes in the css? updates to the class names used? form element properties? What about if you physically move element positions on the page or change tab order? There are so many things you can change we really can't make the form builder smart enough to identify and maintain all of the edits you might want to do after it has run.
As a general rule once you have run the builder you should use Dreamweaver as the editor. If you want to make changes that are so drastic that you don't mind re-creating the form, then you can re-run the form builder wizard.