What I have understood is that the harmonic sequence I IV V I is very ancient and expresses:
I rest
IV momentum towards tension
V tension
I Rest
Let's set aside the fact that it is ancient...
A cadence, in the tonal system, is a sequence of chords that marks an 'articulation'. It is as if it were a punctuation mark, a comma, or a semicolon, or a period.
There are many types of cadences; the V-I cadence (with both chords in root position) is called a perfect cadence. When you add other chords, like IV-V-I, it is called a compound cadence. But in every music theory book, you will find even better definitions than these.
The "meanings" you have given to the chords make sense but should be treated with caution. They are not always part of "classical" tonal theory, but they are often the result of reflections by specific theorists.
Functional theory, for example, considers chords (and also scale degrees, and thus individual notes) as "functions," meaning as elements of a very precise discourse. There are three functions: tonic, subdominant, and dominant. EVERY possible chord can be indicated as a function, even if ambiguous chords exist... functional theory becomes interesting precisely when it deals with complicated situations, especially analytical ones, in which it interprets functions within complex compositions.
At a "basic" level, the chord on the first degree has a tonic function, as does the one on the third. The dominant chord and the leading-tone chord have a dominant function. Chords on the second, fourth, and sixth degrees have a subdominant function. In reality, things are much more complicated than this, and functional theory interprets many other chords, including those I was talking about (the "foreign" chords).
The rest is substantially correct.
Regarding minor scales, there are essentially three types: the natural minor scale, the harmonic minor, and the melodic minor.
The natural minor scale has a minor third and the other natural degrees, i.e., following the list of notes given by the key signature. For example, a natural D minor will have only Bb as an altered note. The other notes will be natural, as indicated by the key signature.
The harmonic minor scale alters the seventh degree to allow for the formation of a major dominant chord. You will notice, in fact, that in natural D minor, the diatonic dominant chord formed with the scale degrees is A-C-E, which is a minor triad. To allow for the presence of a major dominant chord (i.e., the presence of leading-tone tension), the C must be altered to make it C#. Thus, harmonic D minor will have D E F G A Bb C# D.
The melodic minor scale resolves the problem of the augmented second in the harmonic minor scale. Between Bb and C#, there is, in fact, an augmented second, a very dissonant leap. To avoid this, the Bb is also altered to make it B natural. But since this would completely eliminate the key signature, this alteration is maintained only in the ascending scale. The ascending melodic minor scale will, therefore, be D E F G A B C# D, and the descending one D C Bb A G F E D.