Man I agree on that one bigun, sometimes the clusterfu*ks we walk into almost make you want to walk away. My problem is I want to see that its right, for safetys sake so I usually wade in...lol!
grouch, I read the post you had linked to this one and I found it kind of hard to read (probably just me), but I read one from yours towards the end that seemed to show you were on the right track. Copper is better than aluminum. Longer lasting plus if you go aluminum you have to increase your wire size to equal the ampacity of the copper. There is more chance of corrosion using aluminum, plus it sweats and has a tendency, over time, to loosen in its lugs, causing arcing and a possible meltdown. But there is nothing wrong with using aluminum if thats your thing. Don't know about Ky, but here in Michigan the only way a panel in a separate building, fed from another panel, is considered anything other than a sub panel, is if the out building is connected to the main building by water pipes. If so, then you would run 2 hot wires and 1 neutral from the main panel and then the out building is considered to have its own system needing its own ground rod with it being bonded to the neutral. If no water pipe connection, then you would run an additional, properly sized, ground wire from the main panel, and the one in the out building would be considered a 'sub' panel off the 'main' panel. Now heres one option I haven't heard here, which is perfectly legal, given the right circumstances.
Run a small disconnect (sized for your panel to be fed) which is fed off your existing meter. Do this by attaching the feed wires going to the disconnect from the load side of the meter lugs. This is now a new system, and the disconnect is now your first means of disconnect of that system. So 2 hots and 1 neutral feed the disconnect. Install a ground rod at the disconnect. Run 2 hots, 1 neutral, and 1 grd. from the disconnect to the panel it feeds, keeping the neutral bar isolated from the grounded panel. No ground rod is required at the new 'sub' panel you just fed.
As far as the grd and neutral wires being under the same lugs in your house panel, do you mean the branch circuits, or the wires feeding the panel? If its the branch circuit wires, thats ok if the lugs are rated for that many wires. Some are, some are not. The grd and neutral should be bonded together in that panel already, so no problemo. Can you re-feed the 12's that are in the 2" pipe any other way? Its a big no no to run branch circuit wiring in the same raceway as feeders. I hate it when that happens..lol Like I said before, #4's are
legal (Table 310.15B6) and will work fine for 100A in residential, but 2's or 3's will work too. 4's are just easier to work with and a little less expensive. (these are all copper wires I am referring to) Your bare copper wire size for the wire you run to your grd. rod only needs to be #6 also. NEC is usually updated every 3 years with significant changes in every addition. Make sure whatever book you use is current to the existing code. I'm not trying to be a know it all, just want to help a person get it right the first time. Sorry I ran on like this....
btw, whereabouts in Ky are you? I have relatives in Louisville, and the Tolu/Marion areas.