I got a bit more of Alonso's backstory this evening from the female half of the couple that keep him.
She says they've had him since he was small, but he's a rescue dog and yes, he was abused. Took a long time for him to trust them. That explains the fearty behaviour: the way he used to sidle in rather than approach directly when we first met him, the fear of being hit (especially in the kitchen) etc. He's such an odd little doggy, but I've made it very clear he's welcome here anytime.