Great write up and very comprehensive.
I my set up is similar but different in almost every way
but that's what keeps it interesting and allows for lots of chat round fireplaces on different options.
A few of my differences.
I have carabiners at the ends of my hammock but mine are big enough to go right through the ends of the hammocks so I do away with the short lengths of cordage.
I tie a falconers knot to both trees and then tie the tape to the carabiners of the hammock with more falconers knots. this means if I want to adjust the tension of the hammock I can redo the knots at the ends of the hammock, which will be conveniently under the tarp and out of the rain
(I live in Scotland remember)
I have a ridge line running between the carabiners on my hammock and this is what suspends my insect net (so I have the tabs in the inside of the insect net). This means my hammock set up can be independent of the tarp ridge line and so can be as far away from the tarp as I want. Also in sunny Scotland the only time you are likely to be able to use a hammock without a tarp, you will be eaten alive by midgies.
Of course both of these are the way I did things before I changed to my new method discussed here
http://www.bushcraftuk.com/forum/showthread.php?t=17893
Still not sure which I prefer through.
As to the underbalnket I am a big convert. So much so that I forked out for a down version but the group buy version is very hard to beat. The down one really just has the advantage of pack size and weight.
Page 2 of this thread shows my detailed method for attaching the underblanket. Again this is with my new hammock hanging option but works just the same with my original way.
http://www.bushcraftuk.com/forum/showthread.php?t=15711
A few general comments.
Climbing tape is worth every penny in my opinion. I have seen a lot of people use webbing of various sorts but have yet to see one that does not stretch at least a bit. When you use climbing tape your bum is the same distance from the ground when you wake up as when you went to sleep
My two daughters can fight to the death in my hammock set up and I know whatever else happens the tape is not going to break.
Tarp length. I think for group buy hammocks 3.5m long is the ideal length. 4m is good and what I use but 3.5 is my kind of minimum. In saying that I have used 3m tarps lots of times and in really bad weather in fairly exposed spots and stayed dry but I was not very confident. Width wise 2.5m would be fine but I like the extra room 3m gives me.
Now just to clarify I am not criticising Red's way of hanging the hammock and I am sure Red does not think I am. I am just giving alternatives. There is no one right way to set up a hammock
Of course there are thousands of wrong ways