Can't sue him because there's nothing that requires him to sign. The ATF says that a CLEO sign off is required for paperwork done by an individual but there is nothing saying that the CLEO is required to sign. Being that you have several alternative routes, you would have to exhaust every one of them before the ATF would try to work out an alternative.
Agreed, but it might be fun to challenge him to produce his evidence that you would mis-use the weapon. The form only asks him to state that he is in possession of NO evidence that you will act the fool, not vouch for you.