Axel-
I'd be more than willing to try to help you with what your asking. But I have no idea what you are asking..haha. I'll try to answer what you asked though. The first question about playing a C major scale over a solo that isn't in C major. In theory this can be done, and I'll go to when you said you would start out a song with an A5 chord and play a lead in C major. If you understand modes, or just relative major/minor. You would know that you were actually playing an A minor scale over the A5 chord. Since the A minor scale and the C major scale have the same notes, and it's just easily when you look at chords:scale/modes to match the root. But when your soloing over a group of chords in the same key, you can just solo in the root key scale. Although that can be even more boring, and I'll show you a simple example later.
Modulation is going from playing something in one key to playing something in another key.
A minor 'to' C major = relative minor/major modulation. Can be looked at as playing in a different mode but since modes don't have key signatures. It's not written that way. Like there's no key signature for writing a song in a harmonic minor scale, although it is done.
A minor 'to' B minor = foreign modulation. To do this, you need to find a common chord (pivot chord) that is apparent in both keys to make a smooth transaction to the following key. In this case E minor is apparent in both, as v in the A minor and iv in B minor.
Modulation allows for the movement to any chord you wish in a chord progression. In the end, modes play a very small role in modulation. Modes are written for harmonizing over a chord. If a song is written in a mode, the key signature is often itās relative major or minor. But yeah, modes are used in solos and leads. A better way to look it might be melody.
Here's an simple example of how to do use a mode. (cut and paste in notebook)
Here's a chord progression in the key of C major.
F major / C major
Now say you come up with a little riff (melody) like this in C major over the F major chord.
e:--------------
b:--------------
g:----------4~--
d:--3-2-5-3-----
a:--------------
e:--------------
Now you wanted to play the same harmony over the C major chord using the C major scale still, it would be.
e:--------------
b:--------------
g:--------------
d:----------3~--
a:--3-2-5-3-----
e:--------------
Now if that's not what you wanted. Say you wanted to keep the same relationship (intervals) of the notes, like this.
e:--------------
b:--------------
g:--------------
d:----------4~--
a:--3-2-5-3-----
e:--------------
You should know that you did leave the C major scale, cause that last note F# is not in the C major scale. What you did was you played the C lydian mode instead of the C major scale. Knowing that the lydian mode is based on the IV chord in the major scale (in this case F in C major scale), you took the harmonic value you had with the F major chord and continued it with the root chord C major. Put them together in scale succession and you end up with F lydian then C lydian. Very common in music. Hopefully your seeing the concept, and you may be understanding modes a little more. This was just a beginning look at it. If you think about it for a minute, you can probably think up different ways to work with or manipulate it.
If you happy with what your writing without using much theory. Great! I would ride with that man, but understanding more about theory can open up a lot of doors and further your perspective on music. Later on.
"My whole life is a dark room...ONE BIG DARK ROOM" - a.f.i.