The resonance structures need not all carry the same weight. For the benzene and allyl cation problems they obviously do because of symmetry, i.e. all bonds are identical. For general situations there are some rules. Structures with more covalent bonds carry more weight. Increased charge separation decreases resonance structure contribution.
Resonance can also occur via the s system - hyperconjugation.
Spin density is transferred to the H atoms. This can be seen in esr. The odd electrons coupled to the H's. There are several very real problems with resonance theory. Although one particular resonance structure can make a minor contribution, a large number of them can add up, obscuring what one would like to be a simple picture.