Excellent questions!
I'll start with nr 2:
No, the basic tension C-G7-C will remain as an "over-all tension", no matter how many temporary dominants you'll throw in. It will not be affected. It will be proven by the fact that it will not sound finished until you return to C again.
Try playing this, and you'll see:
C - B7 - E7 - A7 - D7 - G7 - C
B7 = B D# F#
A7 = A C# E
D7 = D F# A
This sequence contains no less than five dominants. Try resolving the tension at any point, like:
B7 - Emajor
E7 - Amajor
A7 - Dmajor
D7 - Gmajor
That is, take away the minor 7ths from either E7, A7, D7 or G7, but keep the minor 7th in the chord preceding it, then stop right there. It will sound unfinished, the tension is not resolved. This way you'll discover that the tension will not be released until you come all the way back to C major again.
Even when the music modulates (leaves the main-tonic and enters another main-tonic) to another key, the overall tension will remain.
In Rondo Alla Turca, for example, Mozart sets off in the key of A minor, but ends the movement in the key of A major. Those are not the same keys. So what makes it sound like it's finished?
Well, the Sonata starts in A major in the first movement. About 25 minutes later Mozart finishes the whole sonata in A major. This over-all tension of the basic key of the composition can easily remain for even such a long duration of time! It will be apparent to the listener as well, even if he doesn't know music-theory, he/she will just "feel" it in the air.
For this reason, playing Rondo Alla Turca on it's own will not be the same experience as playing the whole sonata with Rondo Alla Turca as the last movement. Mozart was extremely sensitive to this kind of over-all tension and release.
Question nr 1:
No. First, there's a hierarchy. The basic key of the composition will remain the same! This is what you'll see already in many titles: "Sonata in A major", "Waltz in C# minor" etc.
Next, to leave the basic key to enter another, temporary key, there are almost as many solutions as there are compositions!
Two examples:
Beethoven's Sonata in G major Opus 31 nr 1
He starts the first movement in G major, but within a few seconds he modulates to D major. So very quickly we have a new "main tonic", a new key. But this lasts even shorter. Hardly has he arrived at the new key when he suddenly and drastically changes the key to F major! The time spent at each key is so short to not really being an issue here, but still it really does move to those other keys. But most of all, since G major was introduced at the beginning, this will remain as the over-all key, and the movement will end in G major after maybe ten minutes, as will the whole sonata after maybe 25 minutes!
Robert Schumann's Fantasie in C major, never even starts properly in C major, but the whole first movement slowly modulates towards it. By incredible magic he "establishes" C major as the over-all key, but doesn't really arrive there until the movement finishes after maybe ten minutes. In a sense, this is the opposite of the Beethoven example above, because he takes such a long time to get to the main tonic, but has somehow mysteriously managed to establish it already from the beginning. So, if the Beethoven is extremely short-time, the Schumann can be thought of as extremely long-time.
This is of course not so easy to understand. I don't think it should be "understood" in written words primarily, but rather by playing and finding out with one's ears on a keyboard.
In short, the main key is established from the beginning, and will never be abandoned!
Temporary keys can be established slowly or quickly!
I'll see if I can come up with playable examples...