I think the bigger problem is actually the great big slipper tank which is quite vulnerable, bulky and large... and apparently expected to be carried during combat in this scenario you are laying out. That is foolish.
Because the Mustang could fly 900+ miles on internal fuel (in protected tanks) and do so at close to 400 mph. That way it could fly out to a target say, 600 miles away,
drop it's external tanks when attacked, fight for a while and then fly home with fuel to spare.
Lol! I never made such an argument - I am saying that the SPITFIRE could not be and never in fact was a long range fighter escort. I know it's always tempting to win an argument by claiming that somebody said something foolish that they didn't actually say, but let's not go there.
As for flying with unprotected fuel tanks, it wasn't normally done - even by the Japanese, after 1943... and for good reason. There is a big difference between an aircraft with a few holes in it (even big holes from 20mm cannon) vs. one that has caught on fire. The former can and does fly home, the latter very rarely survives (sometimes fires do go out of course).
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That is a 20mm cannon hole, one of 5 this aircraft received, but the pilot made it back home due to the armor and strong structure of the aircraft. This very likely hit the fuselage tank.
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Same with those - shrapnel from the lower cannon hole almost certainly pierced the main wing fuel tank.
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Also a 20mm hole, right behind the armor plate. And right above or into the rear fuselage fuel tank.
If you assume it's Ok to not have protected fuel tanks "because four engine planes are more important than single engine" then just take the self-sealing tanks out altogether, that would seriously reduce the weight and increase fuel capacity at the same time. But that vastly increases the vulnerability of the pilot.