About your pump, if you've cleaned the wet end and you know that the impeller is in good shape and is turning easily on the shaft, there's not alot more you can do. You have to be sure to remove all traces of scale from the impeller and shaft, using vinegar or diluted muriatic acid. You probably know this already, and if you've done it, your next step is to call little giant, they area U.S. company so there's a chance you could send them the pump and they'll rebuild it. IMO they're medium quality externals; way better than mag pumps but not as good as iwakis or pan worlds. See what little giant says; they may rebuild it cheap. If not, go for a pan world. The genX mak4 is only a little more powerful; it actually runs on less power, but has 3/4" out rather than 1/2". Both of these pumps running correctly would put out more flow under any sort of pressure than a mag 12, so don't worry about that being too much pump.

About your venturi, I'm not sure how yours is set up exactly, but the general idea is that water flow speeds up in the area of the air inlet, creating a pressure drop which draws air in. It's not uncommon for there to be some whistling or some sort of air noise. You can quiet that down with a foam cover. (like a foam windscreen on a microphone) If you cover up the hole, the water gets clear, right? then you take your finger off and it get bubbly again? If so, it's working, and I suspect what you feel is not air coming out of the venturi, but just some turbulence right around there.

What I can't figure out from your photo is: 1) What's the white fitting up high on the tube? I would have thought that's where the water in would go. 2) Where the output is, it looks like there are tons of bubbles; does this skimmer drain microbubbles into your sump? I bet there's an elbow inside the tube at the output so it draws from the very bottom of the tube, but it still looks very foamy to me. This is one of the reasons I build a box under the tube on my skimmers.