First question:

There are actually quite a few ways to look at it. And if your going to be a songwritter, you should experiment with different things and find what works for you. Music theory is only theory (not fact), so it doesn't give you a definitive answer to writting the perfect song. Always remember this when you learn different music principles, they are a guidance not an answer. Used strictly and incorrectly, they can hinder you and you don't want that. What I'm saying is keep an open mind and try different things using your ear as a guidance, then use music theory to understand what you want and how to accomplish it.

Ok, when harmonizing over a chord progression. There are 2 ways to look at it, you can use one scale over the entire chord progression. The other is you can use a different scale over each individual chord.

I'll explain the first one, which is more common and easier to use. When you use this method, the first thing to do is find out what key the chord progression is in. The one you wrote is in the key of A minor. I explained in the first post how to identify, if you need more info, let me know. Anyways, since the chord progression is in the key of A minor, you can harmonize the entire chord progression with the A minor scale. This is a good idea when your writting just a simple catchy melody, that you want to base the whole song on. An example is the Joy Division song you liked. Also, the singing part uses this method alot.

The second method, which is using different scales over different chords, is used more with solos and non-melodic leads. Especially soloists and big players like Vai, Malmsteen and guys along that line. It's also more advanced when it comes to understanding them and how to use them. Here's the idea on how it works. Taking the chord progression Eminor - Bdiminished - Aminor, and playing the following scales over them.

A minor scale: A, B, C, D, E, F, G

Watch how it changes:

Using a E harmonic minor scale over the E minor chord, the notes would be: E, F#, G, A, B, C, D.

You notice how the E harmonic minor has the F sharpened (F#), when the A minor scale has it just as F. This is done to harmonize just that particular chord and not the over-all chord progression. So as you go though the chord progression and change the scales used, you will get different effects over different chords. Each effect will be different, depending on which scale you choose to use.

Choosing the right scale, goes back to understanding what notes make up a certain chord. Take the E minor again, the notes are E, G, and B. So any scale with the root "E" and those notes (E G B) in them, will work over that chord. Believe me there are ALOT, here are some for example. E minor, E melodic minor, E Hungarian minor, E minor pentatonic, just to name a few. So when it comes to choosing which to use, you must first understand how these scales sound, and what kind of effect your going to get when you use them. Here's an idea, the E melodic minor is going to give you a melodic effect, and the E Hungarian minor is going to give you more of an exotic effect. If you are interesting in this path, start learning these scales and practicing them with chords. I'll tell you this, the more scales you know, the more riffs, melodies you can come up with. Definitely a benefit. :)

The second question:

When only 2 notes are involved harmonically (chord-fashion), they are called intervals. Three or more notes is called a chord. Identifying and naming them is quite easy, it's just like counting with letters. The first chord in your example starts with a A and the second note is E. So A would be 1, B would be 2, C would be 3, D would be 4, and E would be 5. So that particular interval would be A5. A "5" interval is a major or minor chord with the 3rd left out; the octave is optional. It's the most common of the intervals, especially on distorted guitar. The exception is with the 3rd chord "9/12", which is by chordal theory an Augmented chord which is written as "A#5". The "#5" is actually the "b6" in the A minor scale, so it's not usually written as a "#5". "Iron Man" consists of all "5" chords.

The theory behind intervals is pretty much the same as chords. This whole music understand of chordal structure and key signature is a solid fact in music theory. The change which is slight in the case of interval chords is when you might play a chord progression like Eminor - Bdiminished - Aminor. It would just become E5 - B5 - A5. If you know how a diminished chord is constructed, you would know that the 5th interval is actually flat, but a "B5" is not a chord. So most of the time, that science is altered. Here's a tab to show you:

progression using the chordal system
E:-----------
B:-----------
G:-----------
D:--9--3--2--
A:--7--2--0--
E:-----------

same when using "5" or "power chords"
E:-----------
B:-----------
G:-----------
D:--9--4--2--
A:--7--2--0--
E:-----------

So when using power chords as your chord progression instead of major and minor chords. The chords are all "5" chords and the roots are based on the key your using. Here's an example of every "5" chord in the key of A minor.

D:--2-4-5-7-9-10-12--
A:--0-2-3-5-7-8--10--

Hope this helps. Latr.