The arming distance would be much more than 30 yards. 200-300 yards would be more likely. The mk 7 torpedo having 326 pounds of TNT would do considerable damage to the submarine if it detonated 30 yards from the submarine.

The problems with a "malfunctioning" torpedo would be more concerned with the propulsion and guidance systems than the explosive system. The warhead just sits there until a mechanical trigger sets off a chemical reaction.

However, the propulsion system, if activated may put such stresses on the interior components that they would not be able to be reused in the field. There is considerable heat and pressure involved and some of the components may have been designed to work once.

Perhaps one of the dangers that Ballard was concerned with was not the warhead exploding but the alcohol in the propulsion system catching fire?

Clearly a submarine was capable of conducting maintenance on a torpedo, but the question on could they repair would be dependent on what needed to be done. What is the difference between maintenance on a system and repairing a system? Depends on what needs to be done.

In reading Iron Men and Tin Fish as well as Helions of the Deep, torpedoes were pretty complicated machines and I doubt that anyone outside of depot could do "repairs" on torpedoes.