Measurement of the B^{0}→π^{-}ℓ^{+}ν and B^{+}→η^{(′)}ℓ^{+}ν branching fractions, the B^{0}→π^{-}ℓ^{+}ν and B^{+}→ηℓ^{+}ν form-factor shapes, and determination of |V_{ub}|