Im so sorry about this, but I want some confirmation on whether im correct.
1. You can zoom right in, focus and hold the shutter then zoom out and it stays in focus? This makes it easier to get a good focus on the eyes for example.
As Bob says, some zooms will hold focus and some won't. But I'm not sure many people use this technique nowadays - it was an old manual focus technique. Modern AF is very accurate at all focal lengths. If you have some older manual focus zooms, the best thing to do is to try it, but if you're going to be that critical in portraiture the chances of your subject staying that still while you do it are even more slender than your depth of field.
2. Shallow depth of field is accomplished 2 ways, by a wide aperture and a long focal length. So a lens, for example a 70-200 f2.8 will have a much shallower depth of field at 200mm f2.8 than at 70mm f2.8
Not quite true. Longer focal length lenses only deliver less depth of field when they magnify the image. If you frame a portrait at 200mm, you will have a certain amount of depth of field for a given f/number; if you then zoom back to 70mm but also move forward for an identical framing, at the same f/number depth of field will also be the same. It's the magnification that changes the depth of field, and you can alter that either by changing focal length, or by moving forwards/backwards.
Focal length in itself does not change depth of field. It is a combination of factors, including f/number, image magnification (focal length and/or subject distance) and format (sensor) size.
However, longer lenses often look like they give less depth of field because the angle of view is narrower. This means that you tend to shoot from a greater distance and correspondingly you will get less background in the shot. This helps to make the subject stand out more, giving the impression of reduced depth of field even though technically it's the same.
The only other thing to note is that if you frame a shot on full frame, and frame an identical set up with a crop format camera (with a shorter focal length lens to compensate) then you will get more depth of field with the crop, to the value of about 1.3 stops (1.6x crop).
Put a few numbers in to
www.dofmaster.com and you'll get some exact comparison figures.
