With you being so uncertain about the builder and the runoff potential, it might pay to get a county agency or similar third party to give you the size pipe needed for your specific pond. The pros here seem to know a lot of the names of these organizations that you could contact. Perhaps they will chime in?
With the anti seep collar, I have never heard of anyone not recommending it. People might opt not to use one, but don't see how it can hurt to have one. Whether you use one or not, with your pipe being near the surface of the dam you would only lose two feet of water to fix it later if needed.
I am not to clear on why you think a siphon is buried more than other drain systems, and why you wouldn't be able to tell if it leaks? Sure the inlet goes deep into the pond, but the rest is similar to any others, at least that is my understanding.
