Chaz, you have a good build and a solid theory behind it. I understand what you are trying to do, how you want to use War Horn to panic enemies and then use your passives to auto-critical these panic'd enemies and lower your cooldown of Rampage (specifically) in the process. It makes sense. I am glad War Horn works for you and you enjoy using it in this manner. War Horn has other benefits which do help the team.
However, War Horn is not for me. War Horn slows down the rate at which I can kill things with my Reaver and lowers my DPS output. I view this as bad. For me, War Horn is at best a situational active skill that I would only use to escape combat and not to use to initiate my offense attack chain. Furthermore, you can not consistently use War Horn to completely reduce the cooldown of Rampage like you suggest. You are really selling the passive Flow of Battle more so than you are War Horn.
As you said above, "Panic causes enemies to run around stupid and this is no problem for you to chase down because... you lunge forward toward targets"
Actually, causing them to run around stupid is very much a problem for me, especially if I need to get 15 criticals in a 6 second window in order to fully cool down my rampage like you suggest.
War Horn cause all enemies within 8 meters to panic and flee in all directions for 6 seconds. The enemies do not all go in the same direction. They scatter. As you chase 1 down in one direction you are taking yourself further away from the other enemies that scattered in the complete opposite direction. Rampage lasts 10 seconds and has a 25.5 second cool down. You would need to chase enemies all over for 6 seconds and have 15 criticals in this time to completely reset Rampage. Actually you have 10 seconds to score these criticals, but in seconds 7-10 of rampage you are dependent upon critical strikes causing panic among more enemies to continue the auto-critical chain. If you War Horn and then Rampage, the enemies will get further away from you in the seconds as you cast Rampage. If you cast Rampage first and then War Horn you are wasting time of your Rampage casting War Horn instead of killing.
When I play with my friends or other players of the same skill level and the entire group of enemies is panic'd, I can at best kill 5 of them before my teammates finish the rest of them off. 5 critical 1 shot kills reduces the cool down of Rampage from 15.5 seconds to 10.5. Playing with the same team, same enemies, but they are not panic'd and without relying on auto-criticals, I generally will kill 8-10 of the enemies in the same time frame b/c I am not chasing panic'd enemies around and they are all grouped together for easy killing. And then with my level 20 amulet that gives +15% cool down, I will have ~12 second cool down on rampage.
The true benefit of War Horn is as an "O ****" button to be used when you have overextended yourself in combat and you need to buy yourself 6 seconds to escape and survive. Hence why you said above, "War Horn provides crowd control and increases survivability."
And finally.. Why do you need Rampage to be back instantly if all the enemies are dead and it takes you 15 seconds to get to the next group of mobs?