Let me start by saying that there are many effective ways to build muscular endurance. What we showed in the book was one simple and very well tested method. As with many subjects we’ve expanded on the topic on the website where space is not a factor. You’d be surprised at how much we had to leave out of an already long book to make it pass muster with the editor.
Now to your questions:
1) See above comment regarding the variability in methods to improve ME. In the book we were trying to give a simple and tested method that would not hurt anyone. On the site we can go into more depth about what is a more advanced method. How much weight you use is determined by how strong your legs are. I’ve had 130lb guys who needed 70lbs before they felt that their legs were the limiter. I have 180lb guys who felt the leg limit with 30 lbs. The only way to figure this out is with trial and error.
2) 13% will not be steep enough. 20% is prob the minimum and 50% is better. I have used 60% and found it ideal. Needless to say you can’t use a trail unless you have one with big steps like that huge set of stairs in Colorado Springs. You are going to have to find an off trail fall line to use. BTW 45 degrees is 100% and you’d need to be using your hands at that point unless you were on stairs. If you can’t find a steep enough hill then a stair machine will work much better than a 13% trail.
3) If you use the above method of ME with heavy weight going as hard as you can manage then you will not use the same progression as laid out in the book for the more conventional weighted hikes using 10-20% of body weight and hiking in the aerobic zone. I can tell you that Steve and I have both done as much as 5k vert in a workout carrying 35-40 lbs of water. We progressed to that over about 3 months of 1x/week. This was using the conventional type as laid out in the book. Using the Zone 3 weighted method I had David Goettler doing no more than 1000m in any workout and he was super strong both times he was on Shishapangma. You will not need so much volume with this higher intensity. But it’s gotta be steep. He did one workout on the North Face of the Aig. Vert with about 15kg if I recall correctly. No progression is needed with the Z3 type. You just go as hard as you can for say 1hour. When you come back to that hill in a week you’ll pass your 1 hour mark at 45-50 min and so you’ll go higher in an hour.
4) If you use the Z3 hard weighted hikes you will not need much weighted hiking otherwise. Lots of Z1-2 volume between these hard workouts.