That's a very nice video. Thanks for sharing it.
1 - I'm not sure. Maybe someone else can help with that.
2 - I don't know off hand. If you have samples of both, I would maybe make a run of each in some scrap leather with your 9spi iron and see how they look. Then use the one you like most.
3 - I was told to generally have the distance of the stitch line to be about the same as the thickness of the leather. So if your leather is 6mm thick, then have the stitch line 6mm from the edge. If I remember correctly, that was in Al Stohlman's book The Art of Hand Stitching, or something like that.
4 - It's a matter of preference again. In the video he is using an alligator leather, right? What thickness do those come in? Some wallet's are intentionally made a little more beefy. This is a judgement call the craftsman makes.
5 - So in this video he has the inside a little longer, glues it to the outside, then cuts off the excess with a paring knife.
http://www.fineleatherworking.com/leather-tools/high-speed-steel-paring-knife
You could try to cut them both to exact lengths first, however you are going to find it far more difficult to glue the two pieces together exactly right. It's much easier to trim off the excess afterwards as he did. You will find that your edges are much cleaner that way.
Hopefully someone else can help you out with those couple of things I wasn't so sure on.
Best of luck making your version and be sure to post pictures afterwards!