That outlet pipe can drop considerably, a 12" pipe can move some serious water in siphon mode even with a 2' drop from the inlet, I wish you were here, I would just give you a huge piece of road fabric to place under it with a pile of rock for normal flowing. If that thing does go into siphon mode that water is going to be shooting 30'-50' and moving some earth.
On your air vent did he install it so was a couple inches ABOVE the BOTTOM of the horizontal water line pipe ( basically the waterline in normal mode) or are you saying it's above the 12" pipe?
I will add I see a couple things that would worry me, but I'm really nervous about the power of water. If you're throwing rip rap around that inlet then I wouldn't worry much but as it sits now, the inflow through that 12" pipe will eat the dirt/clay away around it and keep eroding it until that pipe is probably sticking out 3'.
The 12" valve is a complete waste of money because you already have the 6" main drain valved, open that and the pond is gone and fast.
The angle of the outlet pipe when in normal mode will not drip out and away from the dam but follow the 12" pipe back into the dam and eat the dirt away until it hits bottom or is being diverted.
If it were me, I would cut that 12" outlet considerably, dig out around it a little and have it as the same height as the 6" pass through, with about 1' of 12" pipe exposed and cleaned well, I would then use purple primer to cover it completely and quickly and while primer is still wet cover it completely in medium dry glue, then take some sand and cover the glued area with sand and start to Press and rub the sand onto the pipe as the glue sets, the next day you will notice you have a 12" emery cloth that you can pour concrete against and get great adhesion and with a couple bags of concrete, trowel in a nice place for the water to flow away from the dam.
