1 - How do those siphons work? Can't really wrap my head around the fluid dynamics involved.
They work real well OH, you mean how to they WORK? If you will look at the photo of the, you will see that the long end goes in the F/T and then up and over the end of the F/T into the sump. The end in the sump is looped back up to the desired height to allow them to draw water from the F/T into the sump. When the water level gets below the upward end of the loop, they cease to siphon water into the sump. When the water level gets much lower, a sump electrical switch with a float shuts off the power to the pump.

2 - What is the purpose of the side braces on the GB? It seems to me that they are built in a cantilever setup and do not offer much structural integrity.
The 4" X 4" post there are more shallow in the ground so I braced them. The piece that you see running at an angle actually is braced against another piece (you can't see) running at a 90 degree angle under ground.