A) The zodiac chart and 12 houses are based upon the Earth centered (Geocentric) Astrology. The Ascendant/Descendant, the horizontal axis, is the Horizon. The Ascendant is where the Sun was rising when you born and the Descendant where it was setting. This axis divides the chart into the Northern (night, bottom) and Southern (day, top) Hemispheres. The MC/IC axis or the vertical axis is based upon the Meridian and divides the chart into the Eastern (rising, left) and Western (setting, right) Hemispheres.
You can also read charts when they are Sun centered , which is called Heliocentric Astrology, but it is not a common thing to practice. Most Astrology today is based upon Geocentric Astrology.
http://www.astro.com/astrology/in_house1_e.htm
B) Years of research by Astrologers stemming back to the days of Babylonia.
The History of Astrology -- Another View written by Robert Hand
http://www.robhand.com/histintr.htm
C) Mercury and Venus have been found through thousands of years of research to govern more than one sign. Mercury rules Gemini (the 3rd house) and Virgo (the 6th house). Venus rules Taurus (the 2nd house) and Libra (the 7th house).
D) You are correct. Using Uranus, Neptune and Pluto in Astrology delineations is relatively new. Uranus was discovered via telescope in 1781. Neptune was discoverd in 1846. Pluto was discovered in 1930. Uranus co-rules Aquarius along with Saturn, Neptune co-rules Pisces along with Jupiter and Pluto co-rules Scorpio along with Mars. Interpretations in the past were less complete until the discovery of the outter three planets.
For more info on ancient Astrology go here to Deborah Houlding's Sky Script website:
http://www.skyscript.co.uk/index.html
E) The planetary and luminary motions influence us on Earth, but we don't know exactly how it all works. It just does.
Bob Marks article Astrology for Skeptics:
http://www.bobmarksastrologer.com/skeptics.htm
Questions 8: “If the astrological influence is carried by a known force, why do the planets dominate?
“If the effects of astrology can be attributed to gravity, tidal forces, or magnetism (each is invoked by a different astrological school), even a beginning physics student can make the calculations necessary to see what really affects a newborn baby. These are worked out for many different cases in Roger Culver and Philip Ianna’s book ‘Astrology: True or False’...For example, the obstetrician who delivers the child turns out to have about six times the gravitational pull of Mars and about two thousand billion times its tidal force...”
Answer: So who says that gravitation or tidal effects are the forces that are operational here? Yes, some astrologers have wrongly postulated that they are what makes astrology work. But the fact that they were wrong in identifying the source does not show that there is no effect. And this was done by individual astrologers, not “different astrological schools” as Dr. Franknoi suggests.
And once again, Dr. Franknoi is putting the cart before the horse. Instead of investigating to see if there is in fact an astrological phenomenon, he says, in effect, that there cannot be because he can’t think of a good reason why there should. In a similar fashion, Simon Newcomb, a famous mathematician, “proved” that a heavier-than-air machine could not fly. He, of course, was proved wrong by two bicycle mechanics who did not read his “proof”. Subjectivity like this has no place in science. The proper scientific procedure is to first check if something is so, and then to find a reason why.