When exactly can I break an auto double? (On startup+active+recovery?) a manual? (Only on histop?)
Correct, you can break an auto-double at any point (startup, active, recovery; some "guess breaks" often hit the startup of an auto-double and appear to break before the offense has moved at all) but a manual only on the hitstop. Even if you predict the manual correctly, it can be hard to hit a very small hitstop window, so when used in combos, manuals get 4 extra frames of hitstop that they normally don't get in the neutral game to help people hit the small window more reliably.