I haven't watched your video yet Chris but hope it is all straightened out. For me, raising the level in the overflow was absolutely key to making it quiet. Otherwise that 3,000 GPH of Jebao flow made allot of noise even in a 4' wide overflow. Raising the level was key for noise.
The second key was the depth of the full siphon in the sump. If it is too deep, sometimes the full siphon doesn't start right away.
I guess the third thing that helped was making sure the full siphon is quite a bit deeper than the durso. The full siphon needs room to work without the durso kicking in right away.
You have done your homework so I doubt I am adding anything, but wanted to throw that out there.
I had problems with fittings (t and cap) being to tall also, I just cut them down (carefully) with a miter saw. Use a piece of pipe to hold it when cutting, keep fingers away!
This can be absolutely dead silent. There is absolutely no noise from my DC 12000 now yet it is moving allot of water. All I hear are the powerheads pulsing on wave modes.
Good luck!