Some observations:-
Interestingly the P-51 Mustang was originally developed to British requirements. Initially, the aircraft was a competent effort but was deemed only suitable as a ground attack platform. What transformed the Mustang from another average 'plane to the superlative and iconic WW2 weapon was replacing the American Allison engine with the Rolls Royce Merlin unit.
America did not just "rough up" the Japanese and the Pacific theatre was not just a sideshow. Britain was heavily involved in the far east but due to a series of monumental blunders rendered itself impotent and a spent force. Only the USA had the resources to wage total war on two global fronts. Even Russia was unable to deal with the Japanese in any meaningful way until the European Axis bloc had been taken care of.
The rest of the argument is dependent on "what if" speculation. What if the Japanese had not bombed Pearl Harbour? Could Britain have managed without help from the USA? There are so many permutations that its impossible to tell. I will say that the British, when backs are against the wall, can be ruthless and adaptable. In global conflict its all about attrition. The side that runs out of manpower and resources first is the side that capitulates. Britain is blessed in being an island nation and whilst the country was financially drained by WW1, it not only had a natural moat around it borders, it could also call upon the natural resources of the Commonwealth. All it had to do whilst rebuilding its strength was to ensure the oil supply, which is why North Africa was so important.