Since I went to school for engine rebuilding and automotive specializing in general, especially the performance end of things, I am well aware of how the system needs to function. Maybe I should grab one of my engine books to bring along to Branson since pictures speak louder than words and it may not seem clear. Here is how it works...... When the engine has vacuum in the intake manifold ie idle, light, part throttle conditions airflow is drawn through the pcv vent hold in the throttle body which is the one that passes through the intercooler and into your modified/passage machined lower intake manifold to work with stock pcv type operation. This allows fresh air to be drawn through the crankcase and also prevents you from putting a larger vacuum on the crankcase which will make the noise you mentioned and also try to suck excessive of oil vapor into the engine. ALL engines have blowby to some extent and must have fresh air pass through the crankcase to allow it to vent off. The other end of the system is your PCV valve that is in the blower housing. This is a calibrated bleed so to speak that allows engine vacuum to pull the crankcase pressure/vapor through the pcv valve so it wont build up. This is vented into the top of the inlet of the blower housing and this is why you see wet rotors and lower intake as it is thousands of miles of pcv operation which will have a very fine oil mist to it often. The PCV valve has a checkvalve and spring inside it so it will only flow one way, or with vacuum on it. Now once you go to 0 vacuum or into boost the PCV valve no long is working the same and the crankcase vapors are vented in reverse through the passage that goes back into the throttle body. These gases and vapors are carried through the throttle body and all the way through the inlet of the blower. Along with any pressure potentially built up in the crankcase helping push these vapors back out the incoming air across the hole in the throttle body for the pcv passage helps draw them out. Look at it this way.... You got to McDonalds for a pop and get a good sized straw with it. Try blowing through this and sucking through this.... you get the idea. NOW do this with a coffee straw! I think you know where I am going with this, you have a coffee straw to breathe through with your modified-added pcv passage intake. Back to what I mentioned earlier once you go above stock such as spinning much higher rpms and adding more boost you need more ventilation because the crankcase pressure is going to become higher than on a stock car that's system was designed to perform adequately with. Race engines dont car about the whole spectrum and only run breathers on the valve covers. Your question about only the front half of ventilation... The crankcase is common to both cylinder heads and sides of the engine so it really doesnt matter though you often find the inside of the rear valve cover a bit dirtier than the front one. The PCV system was really used standard back in around 1965 from an emissions standpoint as it used to just get vented out of the engine through vent tubes and some cars with a draft tube that would run under the car, then to the PCV system which will send these vapors back through the engine to reburn and let off less emissions and not have raw oil dripping on the ground. I hope this helps clear up your thoughts on how it works.
especially with the lack of problematic PCV history.