Why the M-class cruiser-submarine concept was a dead end

If HMS M1 and M2 had survived, which cruiser-submarine design would have received further development?

Neither; both M-class paths express a fundamentally flawed cruiser-submarine concept.

The cruiser-submarine idea sought fleet speed, gun-based commerce attack, and the ability to submerge when threatened. Using guns against merchant ships appears economical because a boat can carry more shells than torpedoes. The criticism is that the premise is flawed because it combines incompatible roles and exposes the submarine while using its surface armament. The ammunition economy does not rescue the overall design.