i would use two drain lines for sure, at the very least...i like the idea of bulkheads connected to the tank and a separate tank from there to be used as an overflow..i dont understand the want of a CL though...if you have a vortech already assuming a mp20 or 40...it will be more then enough, especially i paired with another one, and likely wont cost much more then buying the CL pump and plumbing,,,
those preformed ponds are ABS i believe...you may be able to find someone who has removed a megaflow from a tank or be able to get a piece of black abs from your plastic supplier to use as a weir...that or just use several upturned elbows and paint them black..you may even take a black pipe and connect the bulkheads in the pond and cut a slot in the top of it to act as a weir...not the best functional solution, but will work, if you make the slot wide enough it should work fine though...i would build a manifold that would connect all the bulkheads (3,4, 5 etc) if you drill the bulkheads at the right level you wont need any further elbows, if the holes are lower though you will need elbows to raise the manifold up to the surface of the tank(pond) make sure all the connections are glued and that you manifold lines up perfectly with your bulkheads...then cut a 1" wide notch in the top of the pipe..