A reed valve engine does not blow fuel back through the carb unless the reeds are leaky, and if that's the case the engine probably won't run anyway.....
My personal opinion is that the trumpet is just another way for 3W to get money from customers...RTK has it nailed with his little box with a hose in it from the cover...There is no trumpet on a BME 110.....
Walbro and Tillotson carbs are made to work with atmospheric pressure...anything that changes that screws up the carb...Big time...
The trumpet, as designed, would work better if facing forward...The drone engines we use for racing had the carbs facing straight forward, the engines were pushers...Air was ducted directly into the carbs...there was a small tube in the intake venturi that led to the cover over the diaphragm...There were no other holes, so the air over the diaphragm was the same as the air coming into the carbs...NO problems....