Mr Powell's analogies are false. In 'soldier boy', the boy is not (usually) the boy of a soldier: he is a soldier; whereas the forces' sweetheart is the sweetheart of the forces. Again, in 'Army, Navy and Airforce Sweethearts', army, navy etc are standard adjectival nouns. Moreover, all Mr Powell's examples exemplify the tendency of adjectival nouns to be singular (cf 'Girl Guides').
A good test in doubtful plural possessives is to substitute an irregular plural. A true and useful analogy would then be 'children's favourite': one might conceivably say 'the child's favourite', but never 'children favourite'. The forces' sweetheart needs its - her - apostrophe. And you, sir, should take 100 lines for dubbing this a matter of punctuation rather than spelling.
EMI Records (UK)
30 AprilReuse content