A quantum computer with real amplitudes is not an analog computer. The two are quite different. Note that a quantum computer with real amplitudes is actually

A quantum computer with real amplitudes is not an analog computer. The two are quite different. Note that a quantum computer with real amplitudes is actually just as powerful as one with complex amplitudes; the complex amplitudes are actually inessential. Also, your answer to the second question fails to distinguish a quantum computer from a classical probabilistic computer. The key here is really cancellation of amplitudes, not the larger state space.