“Slow Horses” returns on Apple TV+ and the misfits and losers of Britain’s MI5 domestic counterintelligence agency — collectively known as the slow horses, a sneering nickname that speaks to their perceived uselessness — find themselves working a case yet again. This time it involves their fellow reject River Cartwright and his far more respected grandfather, the former head of MI5. Once sharp, the old man has become disoriented lately, and when a visitor arrives at his quiet rural home, he greets them with the business end of a shotgun. Blood is spilled and the cavalry is called. Was it all a…