Classical music traditionally uses a combination of the melodic and harmonic minor scales, so that even c minor has a leading tone B in it. The dominant of c minor is G major. Thus, the minor mode has access to both the major dominant V and the minor dominant v. Most of the time, V is used; the main reason to use v is either as a passing harmony, or when the piece modulates to v for a longer time (or some other modulation; for example v is iii in the relative major key).
If a piece in minor modulates to the dominant, this can either be the minor dominant v or the major dominant V. However, in both cases V/v and V/V means the harmony with a ♯4 in it (in this case F♯). On the other hand, the more unusual v/v would have a 4 in it, but there are not a lot of situations where this harmony would come up naturally.
As for the progression I V I (or i V i), whether this is prolongational or structural depends on external factors. One of the key ideas of Schenkerian analysis is that a lot of pieces are governed by an overall structure that boils down to I V I, so this is definitely not prolongational. But if one of the harmonies is inverted, that changes everything. For example the textbook progressions I V⁶ I and I V⁶₄ I⁶ are usually understood as a prolongation of the tonic.
The cadential six-four should be read as a suspension of sorts. The dissonant notes (6 and 4) have to prepared, for example held over or part of a downward stepwise motion. They resolve to the consonant 5 and 3. Note that this type of suspension is possible with many other chords (and you can find examples of this), but the one on the dominant has become so common that it got its own name.