2. Single length, but had to change number of links....that's in the dual sprocket thread.
3. 13/50 and 13/55
4. (5.) Yes, it was definitely worth the effort, because I ride such a wide range of altitudes, from below sea level to 13,500 feet.
However, I ride the 13/50 about 98% of the time because it fits my home altitude and riding habits almost completely. The only time I changed to the 55 rear is when I went to Colorado and was riding mostly between 9,000 and 13,500 feet.
It takes me about 15 minutes to change, but that is only because I want the chain guard on at all times and I have to use a couple of washers to get it moved outboard to clear the chain on the 55. Otherwise, I could do it in 10 minutes. I also changed the main jet down to 122 at the same time. With that set-up I got up some pretty steep grades at 13,000 feet in first. I actually had to rest partway up, not because of power problems but because it got so hot crawling in first.
So far, I have not seen any increased chain wear, but I put the 55 on the outside, so the 13/50 is pretty close to parallel. I wouldn't change a thing.
Edit: After reading Admirals post, I should say that I need to be able to ride on the highway at 60 for a few miles on nearly every ride when I'm not on a camping trip, so I have exactly the opposite experience. I've only used the 55 rear for maybe 200 miles out of 2,000 so I have not seen front sprocket wear. (yet.)