I just made a flat tarp recently too, and I really like it so far. As for your questions
1)an 8 X 6 tarp is not huge by any means, but it will be fine for what you want to use it for, in an A frame setup, there is a bit of space underneath, definitely plenty if you also use a bivy.
2)If you are going to have to use two widths of silynlon, you might as well just make two 3' panels so that the ridgeline is down the middle. I am not sure if having the seam off to the side like that would cause any issues, I wouldnt think so, but I could be wrong.
3)Like i said in that last question, Im not real sure about that one, I dont think ive ever seen a tarp with a ridge seam down the side.
4)You have plenty of tie outs, I just sewed the 8 basic ones on mine, and it works fine for the pitches I have practiced recently.
5)I did the same thing on mine, and based it off one that a friend had made. Both of ours have 4" traingles, and 4" circles in the centers. He has his tarp for 2 years and its still holding up fine
6)I dont think so, one thing you might consider though to simplify this project alot is to make a 5X9 tarp. This will prevent having to make the seam down the middle, and most silynlon comes in bolts that are about 60-64" wide, so you will have a little over 5 feet width. So for only 9-10 inches less on the width, and a foot longer on the ends, you could have a pretty easy project. Just putting a hem on the outside edges, and sewing on the reinforcemnets/pullouts. This is what I made, and I really like the space underneath it in combination with a bivy. The extra foot is really nice if you are a taller guy.