You will not be able to set the ID/Name of a form element within form builder, but after you apply the form you can use Dreamweaver tools to set the ID/Name to what you want it to be.
If you already have a form created, you can use Form Builder to scan the form and create the form with the ID/Names that you want.
The Header is for labeling a group of form elements. For example if you have a group of text fields for Street, City, State, etc, you can label the group as Address.
The Spacer Text doesn't seem to be working. It is supposed to be the text that separates the element label and the element (like using ":"). I have logged a bug for this.
I'm not sure what your question about radio buttons is. Could you be more specific?