It's been a looooong time since I've used PP but I would be more concerned about aeration if I had fish in there. I think the movement of your filters will suffice.
I don't think you'll need to monitor for 15 hours. Organic material in a pond will make the solution turn from purple to brown much much much more quickly than a barebottom tank. I'll be surprised if you need to redose at all.
Your calculation is correct. Again, I wouldn't worry about the precise volume, especially since there aren't fish in there. When I used it without fish, I simply added some PP til I thought the tank was "purple" enough. I'm so scientific.
I used Prime to neutralize it if I remember correctly . . . so I'd follow the directions of the hydrogen peroxide. I would do a large WC (as much water as you can get out) then dose ammonia.