Don’t know where you got the story from but it has been muddled in the telling.
The Dutch ordered 32 DB-7B and 48 DB-7C. It was 6 of the former that fell into Japanese hands. 4 of the remaining DB-7B went back to the US and the rest of the DB-7B (22) went to the RAAF. The 48 DB-7C on order were completed as DB-7B and shipped to USSR under lend lease. You will find the story here.
There was not a problem with the A-20 as a torpedo bomber. The USSR used plenty in that role.
And yes dropping torpedoes is more difficult. If the torpedo is dropped outwith its proper parameters it could
1. Break up on entry to the water
2. Dive to the bottom
3. Porpoise and head off in the wrong direction
4. In less sophisticated systems the pilot/bomb aimer has to mentally calculate how far ahead of the target ship to aim and in doing so judge its speed and angle. In more sophisticated systems, as employed on British Beaufighter and Barracuda aircraft there was a “computer” to help. In those the pilot pointed the aircraft at the target ship but still had to make inputs to the system. That needed judgement.
In the FAA it was felt regular practice was necessary so a synthetic trainer was created to assist.
Edit You might enjoy this about Soviet use of the A-20. Scroll down for Soviet Navy use as a torpedo bomber.
The Douglas A-20 Havoc/Boston in Soviet ServiceThe A-20 Havoc/Boston twin-engine multi-role aircraft had a laudable service record in World War II. From its brief service with the French Armée de l’air before the Fall of France in 1940 th…vvsairwar.com
And the RAAF were training their Beaufort crews on torpedoes so probably decided that one type was enough at that stage