Rainman,
Question 1
With the top of the vent tube a couple inches above the apex, water will drain from the bottom of the pond when water in the tube reaches the downstream side of the apex. I assume that this type of drainage is not that significant. Is that correct?
With the opening of the vent tube above the crossover pipe apex, atmospheric pressure pushes water up and through the pipe like it was level and open on both ends. Until the opening to the vent tube is block (by rising pond water level) there is no siphon action. Once the tube IS blocked, siphon action starts instantly! In practice, the sudden siphon start will let the vent open and close, making the siphon start and break rapidly. A flat piece of something on the vent tube opening can reduce that by reducing the venture effect at the vent opening (like a swirling toilet when you flush)
Question 2
Most diagrams that I've reviewed have an adjustable vent tube. Can the vent tube be fixed with the top a couple inches above the apex? Also, can some type of "vent hose" be attached to the fixed vent tube if you needed to lower the water level?
as long as the vent only has one opening and no air leaks, it can be made of most anything. The main purpose of the vent is as a vacuum break, to stop the pond from draining dry once a siphon starts.
These are just the very basic principles of a siphon. You will also want to make some sort of "trash rack" on the inlet of the main pipe and vent opening to ensure they never plug up.
If the main inlet plugs, or worse, a stick or turtle gets trapped in a curve, then water will not flow properly and could lead to a dam breach and difficult repairs. If the vent plugs, the siphon will not stop till the pond drains completely, or the lift is too high to overcome and water either vaporizes or the tubing itself collapses from the vacuum pressure.
