Testing the Mark 14 torpedo before American entry

Would 1940 be early enough to restart United States torpedo work and properly test the Mark 14?

Yes; corrected torpedoes might not be ready in December 1941, but could improve early 1942 submarine operations substantially.

A 1940 restart is late but still early enough to expose the Mark 14 torpedo's defects before American submarines depend on it in combat. Systematic testing and renewed factory work might not deliver a fully corrected weapon by December 1941. It could nevertheless produce reliable torpedoes by January or early 1942, improving a large part of the opening submarine campaign. Even an honest prewar appraisal would let crews prepare local modifications and realistic tactics instead of discovering every failure against enemy shipping.