The non return valve is exactly what it says, it doesnt allow the air to return back down the line. I imagine its because if the valve so it works more effiecently but if you have done away with it then its of no relevance to the setup. My pictures i posted show simply what your doing. Instead of the valve opening the DV its done by good old vaccum from the manifold, hence why you have a spring in the dump valve, once the vaccum exceeds the pre-load on the spring it will then pull the valve open and chhhhh it dumps the pressure. With the N249 valve it has no vaccum at the valve untill it opens when told to by the ECU, So if it looses traction when you accelerate it operates the valve and dumps the pressure there and then, hence lack of power. Its a very simple & basic system once you learn how it works, same as the N75 valve.
Its a simple modification that doesnt effect any major components of the engine, its the same as binning an EGR valve, take the valve off put a blanking plate in and bolt it back on, or if your not too botherd do away with the valve compleatly, same principle, either keep the valve attached and bung the ends up or do away with it. You also mention it sucking dirt into the engine, once the hoses are cut from manifold & DV and you bridge the valve, the valve is compleatly isolated and even if it sucks up a budgie it wont get in the engine
Here is the N249 system highlighted red & green
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There is basically what your doing, as you can see the vaccum goes to the valve and when its shut dump valve is close, as soon as the ecu opens the valve then the dump valve opens.