Which brings you to question 3, and the answer is pushing — and it is exactly the technique your question is circling.
Shoot the entire roll at 1600 instead of 400, deliberately, then tell the lab to "push two stops". They develop it for longer, which raises the density of the underexposed frames to something printable. That is how people shoot 400 film in a dim bar.
What it costs, so it is not sold as free speed: more grain, more contrast, and shadows that stay empty. Pushing does not recover detail that never got recorded — it lifts what did. Two stops is routine; three is a look rather than a rescue.
The key point for you: it is a decision about the whole roll, made before you start shooting, because it changes the development.