Do I have to shoot a whole roll of film at the same ISO, or can I change it partway through?

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.

Your model is exactly right, and it is the model most people never get to — the dial talks to the meter, not to the film. Everything follows from that.

So changing it mid-roll is not forbidden by the camera; it just underexposes or overexposes those frames relative to the box speed. Set 1600 on a 400 film and the meter gives you two stops less light than the film wants. Those frames are underexposed by two stops. Nothing stops you. The film simply records what arrived.

The reason it is usually described as "you must shoot the whole roll at one speed" is what happens next: development is a property of the whole roll. The film goes in one tank, in one developer, for one time. You cannot develop frames 1-12 normally and frames 13-24 for longer. That single constraint is where the rule comes from, and it is a darkroom constraint rather than a camera one.

On question 2, what actually happens to mismatched frames is worth knowing because the two directions are not symmetrical at all.

Overexposed — you set a lower ISO than the box, so the film got more light than needed. Colour and black-and-white negative film handle this remarkably well. A stop or two over is often fine, and some people deliberately rate 400 film at 200 for richer shadows. This direction is forgiving.

Underexposed — you set a higher ISO, film got less light than needed. This is where things are lost. Shadow areas that received almost nothing have nothing to develop, and no processing brings them back. One stop is usually recoverable; two is visible; three is mostly gone.

So if you must be wrong mid-roll, be wrong toward overexposure. And this asymmetry is the opposite of digital, where highlights clip permanently and shadows can be lifted — which is exactly the habit that catches people coming from digital.

The practical way out of your actual situation, when you are halfway through a roll and the light drops: leave the dial alone and change something else.

Open the aperture, drop the shutter speed as far as you can hand-hold, brace against a wall. On a 400 roll indoors you are often only a stop or two short, and those are available without touching the meter setting.

If it is genuinely much darker than that, the other honest answer is to finish the roll quickly on anything, rewind it, and load a faster one. A few wasted frames is cheaper than a dozen unusable ones, and much cheaper than a roll you then have to guess the development for.