When applying for US citizenship, after taking the interview with the USCIS and being told that no further interviews will be required, according to the US law, the USCIS has up to 120 days to make a decision on the case.
When there was no decision within 120 days, based on what I have read, there are two possible legal actions that the applicant may take (please let me know if there are others):
File a petition for a Writ of Mandamus in Federal Court.
File a petition for an action under Section 336(b) of the INA, 8 U.S.C. § 1447(b) in Federal Court (The Federal court acquires jurisdiction over the matter; The court may either decide the naturalization application or may remand the case with instructions to USCIS).
For each one of the two possible actions above:
A. Can the (non-lawyer) applicant file independently (without hiring a lawyer)?
B. Does the applicant need to have a US address at the time of filing (assume the applicant stays abroad for a few months)?
C. Does the applicant need to physically be in the US at the time of filing (assume the applicant stays abroad for a few months)?
D. If the applicant currently does have a US address in which he currently resides, does he need to file the petition in the federal district court, in the jurisdiction in which the applicant resides? Or can he file it in any federal court in the US?