Welcome to the forum.
Fish hatcheries will sell to anyone that will buy the fish. Ideally they should work with the pond owner so they don't stock the incorrect fish for the pond owners goals, or incorrect sizes of fish (that may be eaten within a few days of stocking).
Snappers will walk a mile or more to get to a body of water, especially males during breeding season. So, even though you may eliminate some now, different ones may be there next month.
Eggs are probably too small for the turtles. Dead fish, crayfish, etc. will be food. If they run out of food, they might leave the pond to walk to another one too.
Are the turtles you see sunning snapping turtles or are they a different species?
As for a siphon drain, look in the Pond Boss Resources Guide. Maybe someone there can help.
http://www.pondboss.com/resource_guide.asp?c=9 
