(2) Generally, if the box for testifying is not marked, it usually means they only want the documents to be produced now. That does not mean you cannot be re-subpoenaed to testify at a later date. However, rather than simply operating on that assumption, you should get a lawyer and the lawyer should call the prosecutor to find out exactly what the grand jury wants from you. It’s much better to do this than to have a misunderstanding with the grand jury.

(3) As to testifying, your lawyer can speak to the prosecutor and inquire as to whether or not you or someone else who has not been subpoenaed can testify. If the prosecutor declines to let you testify, there is nothing that requires the prosecutor to let you do so, so you can't go into court to enforce your right to appear. You can ask to be allowed to appear; discuss this with your lawyer. In many districts, your lawyer can contact the court that supervises the grand jury and rely your request to appear through the court. Do not contact the grand jury directly; a federal statute makes it a crime to communicate with a grand jury (in writing) with the intent to influence their activities. Prosecutions under the statute are rare, but not unheard of.