Uranium, Lead, Supernovas and the Age of the Earth.

The age of the Earth

I have been thinking about this for a while after I read an article about how an American scientist in his quest to understand how old the earth was had been plagued by lead pollution.  In the end he determined that the earth was 4.5 billion years ago.  The debate about the earth's age had raged on a bit and several different methods were tried and abandoned until radioactive decay was better understood. This article from a 1989 Scientific American gives a good if a bit long explanation of the pursuit of knowing the earth's age.

So as I understand it, the earth's age is determined by the radiative decay of Uranium into lead.  By measuring the ratio we are told that we can know what the earth's age us (provided we find the oldest rocks available).

I see some assumptions at work:

1. That when the uranium in these so called oldest rocks were part of the forming earth that it was very close to 100 % uranium.
2. That uranium's decay rate is universally constant, unaffected by things like gravity, radiation, speed traveling through space, heat or pressure.
3. That the oldest rocks (as we determine them to be of such an age) truly are the best way of determining the earth's age.  

This causes me to ask a few questions:

1. If the uranium/lead ore we find on the earth was originally formed in the heart of an exploding star and given that the radioactive decay of uranium is a constant, then isn't the age of the Uranium/lead determined by when it came out of the Supernova?
2. How can we know the ratio to lead and uranium in contributing rock or asteroid or particle when it arrives to our solar system?
3. How far away is the closest Supernova remnants and how long would it take material to travel from said remnant to our solar system?  Could we determine such transit time with any accuracy? (Not as far as I thought just 1000 light years, so if the material from the supernova traveled at 1% of the speed of light then it would only take about 10 million years to get here, but there is still the problem of slowing down enough to be captured)
4. So then the age of the uranium/lead or as determined by the ratio would be an indication of how long it was floating around in space before becoming part of our solar system wouldn't it?
5. How slow would material have to be traveling in order to be captured in our forming solar system?
6. How is the ratio of lead and uranium affected by passing through the mantel of the earth, should it be either part of earth's original construction or subducted into the earth's mantle in a subduction zone?
7. And finally if the age of the uranium/lead ore is determined by the point it left a supernova, then wouldn't the age of the earth be younger than the youngest piece of ore found on the earth?

My own conclusions:

1. The earth is younger than the youngest uranium/lead ore found on earth.
2. We cannot therefore know with any certainty the actual age of the earth.
3. I would further state that we cannot know the ratio of uranium to lead of any rock meteor arriving before or during the forming of the earth without considering any and all scenarios.

I invite any criticism of my questions or conclusions.  Feel free to explain your reasoning.