I agree that a runner's physique plays a big role here, especially in regards to their mixture of muscle fiber types. At higher levels, I think it is easier to train a 200m runner's endurance than to train an 800m runner's speed. The 400m is a sprint and if you do not have a good enough top speed, you will not win.
Your question about intermediate distances is interesting. I have run 500m races (on indoor tracks) in the past and I rarely saw sprinters attempt the distance. I saw plenty of 200m sprinters run the 400m from time to time, but the 500m was just too much. This is likely because it really is just too long to sprint the entire time. Your anaerobic energy will be nearly depleted by the 400m mark, yet you still have to go 100 extra meters! Pacing, endurance, and strategy become important here, and most sprinters are not trained in these areas as much, but 800m runners are, since the race is one of the most strategy-intensive races in the sport, with the positive split strategy (running the first half faster than the second half) being what tends to get world records.
That said, I think that with the right training, 400m and 800m runners could be competitive with each other at the 500m, and maybe the 600m, but I think the 500m is more likely to be the competitive distance here.
I think the 15sec difference is due to the fact that no person can run at 86-second 800 pace for 86 seconds.