I run a Dart on a 120 and it works okay. I have 3 holes drilled for the return w/ T's inside the tank. If I had a barebottom, I would probably upgrade to a bigger pump. With a 180 I definitely upgrade to something bigger than a Dart. My pump requires 2" intake so I drilled two 1.5' holes and have them T to a 2" into the pump. I drilled two just in case one clogs up a little bit, it shouldnt cavitate the pump.
As far as using the second overflow for a CL, I dont think you will be able to get enough water though it to run a big pump like hammerhead. I would keep both overflows going down to the sump so you can get good surface skimming and in case one gets clogged, the other one will help out.
I would recommend to do a hang on the back overflow so you wont have to drill the tank and in case you dont like it, you can take it out. A oceans motions is the way to go if you want alternating current. It has a motor in it that changes which pipes you want water to come out.
As for the German thing, I dont care how good of flow it makes, it would look hideous in tank seeing that thing take up both sides.