You ask good questions!
An aspect can't perfect if the planets have to change signs to do it; this is seen as an impediment or prohibition to perfection. The only possible exception would be the Moon, if she is in late degrees and immediately aspects a planet from the next sign; in this way she is said not to be Void of Course.
A planet having to change signs in order to perfect *can* be seen as 'a change of circumstances before it happens'. In the case of Venus and the Sun, however, since the aspect doesn't perfect for such a long time, more than halfway through the next sign, we have to ignore this 'rule' entirely: in the eyes of a horary chart, Venus and the Sun will 'never' perfect, at least for the question at hand. The conjunction is just too far away. This wouldn't be considered a fortunate aspect anyhow, as Venus loses all of her dignities in Gemini and so has no power to protect herself from the Sun.
If Planet A reaches Planet C before Planet B does, this is also prohibition. So the Moon reaches a planet before Venus does; unless there is translation of light from the Moon to Venus to the Sun. But again, a change of signs means that the aspect doesn't perfect. If it's not going to perfect in the chart in front of you, it's not going to perfect.
There is a real cautionary tale about extrapolating what will happen 'when this happens to Planet A', especially if it happens only when the planet changes signs. Horary deals with the immediate
, so the fact that a planet reaches another planet's terms in 21 degrees or so, is basically a hiding to nothing. Looking at the present moment, Planet A isn't
in Planet B's terms, and so it can't be considered as a factor.
We can't look at a horary chart like a developing movie plot, in other words. Horary is more like a still-shot than a film; it captures the essence of the situation at the moment you ask the question, because life is meant to be lived moment-by-moment, without the heavy focus (some might say obsession) with what's going to happen next
You can get general trends from immediately-applying or separating aspects; so in this way you can see that you will or won't get the job, for example; or that he will or won't come back to you. But reaching any further than that, to see what the job will be like a year from now, or whether he is in fact going to go back to his ex-girlfriend sometime next March, is an exercise in futility, much like 'guessing' what will happen when such-and-such planet reaches the degree of its exaltation, halfway through the next sign along; there is a great deal of Free Will at play in our lives.
Did that help?