Where to begin
The trouble with protein skimmers is the lack of available comparison testing. There was a long thread in the equipment forum a while back about ways and methods of testing which skimmer pulls out more dissolved organic compounds. We really can't accurately test these things. Too many variables that can throw off the results. If you think about it, there really is no practicle way.
The theroy behing inline skimming is that according to Calfo, proteins tend to float to the surface. As a result, the idea is to deliver that nutrient rich water directly into the skimmer, at a slow rate, and let the skimmer work from there. That gives you "raw" tank water.
Now, not all water can go through the skimmer, so the water that doesn't gets redirected back into the sump that in hopes it will pass through the skimmer at some point. I redirect mine into a filter bag that I have carbon in (I change it often), then through my refugium.
Now some detritus will/does fall to the bottom of my skimmer. That solid material is removed later with a gravel vac.
As for insump skimmers, you are getting that same water, but according to Calfo again, that concentration of proteins is compromised as it enters the sump. Why? I'm not sure. Can it proved? I'm not sure either.
So is it better to run one or the other? I think its just more about how you want to run you tank

I have a way that I like, and works for me, and others (like Tim) have AMAZING tanks with insump units. Your tank is very nice too
Now my opinion on flow through the sump is that it should be slow. All of the flow should take place inside the tank allowing all the detritus etc, to be suspended, but thats the common concensus among those who run BB tanks.
I consider my skimmer to be a major source of waste removal on my tank. I don't feel that I can strip the water of too many organics to the point that I would see any negative results. But I also don't rely on it alone. I run carbon and a filter sock to help out. I also run phosban to keep the phophates low. I think its important to have an all around balance to the system. Some methods are more efficient than others at removing different things.
In your low nutrient tank, I wouldn't expect much production out of your skimmer. Its very large for the application, and I'm sure it keeps on top of your bioload very well. There seems to be a common misunderstanding that a bigger skimmer will pull out more stuff. Thats really not the case! Larger/oversized skimmers remove waste quicker, and then run idle when there is nothing else to remove. See, once the waste is gone, the skimmer can't pull anything else out. It will only work when there is something in the water column. So when you have a very efficient skimmer (like your ER) you can count on it to work hard for the first few months until it catches up, then the skimmate prodution will slow down. If it doesn't slow down, then your skimmer is undersized IMO, and is not keeping up with your bioload.
I wouldn't worry about the air valve. They are worthless. I don't use one.
With wet VS dry skimming, once again there is no real proof about which one is better. Some say wet skimming removes waste quicker, but dry skimming removes more solid waste.... So I do both. I'll run it wet one week, and when I'm bored, I'll switch it over to dry
Well, you asked for it :lol:
This has to be my longest post EVER..... Yeah, I'm a geek. I'm gonna go watch sportscenter now
Jim