Pipercameron,
Note that with your drawing the vent slot/fairing/pipe arrangement was a retrofit item introduced around 1953; so that all the original Chipmunks that came to Australian aero clubs did not have this (the RAF understandably didn't bother to install this on aircraft already ear-marked for disposal). This installation was intended to pressurize the tanks to ensure even feeding. Certainly the RVAC (at least) modified their Chipmunks without the vent to have a L/R/OFF selector.
If the fuel cock is lower than the tanks then it's only very very marginally so. On visual inspection I'd argue that it's at the same level.
And lastly, despite what the Pilot's Notes say, I know of at least one instance where with one tank empty and the other full (surely this evolving situation would have rung alarm bells to the pilot?) the engine failed.