I think the reason that a default value is required is because if the student does not enter a value in the text box then there is some text that can be used if the variable is displayed later in the project. In your case this might be 'no email address was supplied' or something like that.
On the results page, you should be able to add the page text optional property to allow you to add some text at the top of the screen. You can then use [emailaddress] within the text field to surface the variable value.
To add the Xerte project ID to the page text, add the 'surface global vars' optional property at project level and ensure it is checked. Then, on the results page, the project id can be added by including {x_TemplateId} in the text.
Hope this helps