Try as we might, we just can't avoid that pesky physics. When you cut away all the fancy pieces, it all boils down to this... During normal operation, all mechanically actuated underwater demand regulators require displacement of a diaphragm to open the valve that provides air. In scuba, the regulator is tuned so that this diaphragm moves effortlessly in a normal swimming position. On the "hockey puck" style regs (includes Apeks) this is with the diaphragm slightly lower than the mouthpiece, so there's the tiniest little bit of water pressure already on it. When you turn it "upside down" you're positioning the diaphragm above the mouthpiece, and now you have a "bubble" under the diaphragm - the pressure differential has been reversed, and you now have to overcome that half inch or so of water pressure before the diaphragm can move and open the valve to provide air. It's the same reason you can usually stop a regulator freeflow by simply turning the mouthpiece down.
Attempts to overcome this have been made by placing the diaphragm to the side (Poseidon, for example) rather than the front of the reg, and that works - but - now you've just moved the "hard to breathe" position from "upside down" to "on your right side."
So what you're experiencing is "normal operation" - irritating to many, with a little experience it'll be something you learn to expect, compensate for and eventually you won't even notice it.
As for wet breathing, that's another matter and has to do with the placement of the exhaust valves. These are positioned at the "normal" low point in the reg, so any water that gets into the reg is exhausted with your exhaled breath. When you turn upside down for any length of time, water can accumulate since the exhaust valve(s) is no longer at the low point, and you either need to get it there for one exhalation or do a brief purge or forceful exhalation to displace enough of the the water upwards to where the exhaust valve is now located to make breathing "reasonably dry" again.
Rick