Re question 1: you'd typically measure the abundance ratio by putting a sample of your element in a mass spectrometer and counting how many atoms of the different masses you get. This is exactly how carbon dating works: it counts the C$^{12}$ and C$^{14}$ atoms and the ratio tells you how much C$^{14}$ has decayed.
Re question 3: it's difficult to find rocks that have been unchanged since the Earth was formed because plate tectonics tends to mangle early rocks. The age of the Solar Sytem, and therefore the Earth, is calculated by doing radiometric dating of meteorites. For example the Allende meteorite is dated at 4.567 billion years old. We assume that the Earth formed at the same time as the meteorites. The oldest rocks on Earth are somewhere around 4 billion years old depending on whose claims you believe.
To back to your question 2: I don't know of references for the isotopic composition of the early Earth, but I imagine it would be easily Googled. It's difficult to be sure about the isotopic ratios because we don't know what the ratios were in the dust cloud that formed the Earth, however the ratios can be calculated in some cases. For example uranium decays to lead, but of course there is lots of lead around anyway so we can't tell what lead has come from uranium decay and what was there originally. Once again we use meteorites. By finding meteorites that contain lead but no uranium we can infer the original isotopic distribution of the lead. Then by comparing the isotopic distribution in Earth rocks with the distribution from the meteorites we can work out how much lead has come from uranium decay and therefore what the original uranium ditribution was.
