The statements you read in the manual are the classical way of describing the correct exposure in simple words. Many manufacturers used similar descriptions in the past. These statements are generally true for films and have been confirmed by numerous professionals and amateurs over the years. It is an easy guideline for near-correct exposure in most circumstances.

The digital age has changed most of this. But let me address your questions:

measure the Middle-Tone areas

the "Middle-Tones" here refer to the center of the whole range of contrast in a scene. The colour negative films have quite some latitude when it comes to the exposure. Underexposure by one F-stop is undesirable, but never a big problem. Overexposure by two F-stops is seldom a problem. Therefore the idea of this advice is that you put your meter to the "important" part of the scene, that part that you want to show. If you're unsure, overexposure is better than underexposure.

The industry has created "gray-charts" (like the Kodak Gray Card R-27) to simplify generic measurements. These charts have a density of 0.75 log D. (which means that it reflects about 18% of the light) This is the brightness that usually "carries the most content". Put this chart into your scene and measure the exposure on that chart. This will give an "average" exposure which is correct for the light in that scene. However, adaption of this is required if you have mainly whites or blacks in the scene.

Meter on the shadow - develop for the Highlights

This is the general advice to control the contrast of a b&w scene. Traditionally, b&w films are processed and printed by the photographer. So everything is under his control. The problem lies with the sometimes huge range of contrast.

The idea behind this advice is the following: You want enough exposure in the shadows to draw at least something there. Underexposure lets the shadows slip into complete darkness. Therefore, measure "towards the shadows" to get the shadows.

This results in lights that are too white. This can be countered with reducing the contrast in the development of the film. It's pretty easy to reduce the contrast during development by reducing the development time.

overexposure + underdevelopment = reduced contrast

underexposure + overdevelopment = increased contrast

When producing prints from your b&w negatives, it is usually better to print low-contrast negatives which you increase in contrast. The opposite is possible, but renders usually worse results.

If you measure both the shadows and the highlights you get an idea about the range of contrast in your scene.

Maintain overexposure settings for the whole film?

Here the answer is different for b&w or colour negative films. As the colour negative films are usually processed in a lab, you don't have the possibility to influence the contrast. And even if you are able to process them yourself, it does not deliver the desired results. Prints are also usually created in a lab, where each image of a film is printed "with its own" settings. Therefore, it does not really matter. Newer generations of negative films (the ones after the Kodak Ektar 1000) usually have a huge latitude when it comes to the exposure.

B&W films however, are processed by the photographer. The question also applies only to 35mm film (not sheet-films). Since different scenes usually have a different range of contrast, it is usually desirable to control the contrast "per subject". Therefore it is convenient to use one film only for one subject and choose the setting for the whole film.

Overexposure for b&w the same as for colour negative

Generally speaking overexposure is better than underexposure. This is true for both, b&w and colour negative. This is just the general rule. But if you go for the contrast control mentioned earlier, then No: use the correct exposure for b&w. For colour negatives slight overexposure is ok.