Here's what I remember from my Polymer Science class in college:
Every polymer mixture (i.e. bottle of oil) has a variety of chain lengths. A dino oil has a bell curve distribution: a few really short and a few really long chains with the average being the correct length for motor oil. A synthetic has a much narrower curve: very few really long and short ones, almost all of them at the correct length.
Now I forget if it the long ones or shorts ones that burn off in the engine, but those chains are the ones responsible for your dino oil turning jet black in 3000 miles. The TRUE synthetics don't have that problem and hold their properties for much longer.
So for your questions:
1) My engine went at 86k and I ran Mobil 1 from 3000 miles on. I do not blame the oil, though.
2) Use Mobil 1. It is the best for the buck, hands down. OEM fill for Porsche and Corvette.
3) The monomer structure of dino and synthetic oil are identical. Viscosity also. The only difference is the chain length distribution and the synthetics have more of the "optimal" chains, so they won't hurt your engine internals at all. I agree with Toadster's answer to this one, and I have not seen it myself but have heard stories.
4) I've seen something on the forums that was from an oil engineer at Ford saying that you can't go back, but I'm not sure why it would matter. He should know more than me though.
5) I went 5k (sometimes more) and the oil looked fine. It was the filter that I started to worry about or I would have run longer.
I had a really cool Polymer Sci professor who was also a car guy. He spent an entire class telling us all about synthetic/dino oil.
