I'm not sure I understand your 3rd question ..... theaten the structural integrity of the dam area above the bentonite line during overflow conditions?
Spillways are typically designed to be wide enough to handle calculated flows. Water begins to flow over the spillway gradually as the water level increases. Spillways are grassed/vegetated or rock lined for erosion control and soil stability.
Betonite seals are often poor seals near the upper water line or above the water line where the water level fluxuates and the soils are allowed to periodically dry out, crack, and then leaks result. That is why it is best to have a very thick clay liner or wide, tight core trench as a seal for the upper 5 to 6 ft of embankment around the pond. If I had to use a beonite seal in the upper 5 ft of embankment, I would make sure to work the betonite at least several feet back from the water line into the soil in layers with compaction of each layer. You don't want this to leak which is where it is most likely to leak. It will be much cheaper to do it right the first time rather than try to "fix a leak".
You should also realize the possibility that your soils maynot be appropriate for building a pond.
