All this talk of time travel puts me in mind of a novel I read back in the late ‘80s. The 20th century world is on a timeline where Spain is the dominant world power (Francis Drake never managed to burn the Spanish Armada), time travel technology was invented by a monk named Bonaventura, and the protagonist is a cop who specializes in solving and preventing Time Machine-enabled crimes.
This might be a novel that was cobbled together from a series of short stories about the cop. In the last chapter, at the end of his adventure he returns home to 20th century Nuevo Yorke, only to find that time travel doesn’t work any more, Drake did burn the Armada, Bonaventura never invented time traveling tech, and now he’s stuck in New York.
The author might have been John Varley, but I won’t swear to that. (ETA: probably not; none of the titles in the bibliography on his Wikipedia page rings a bell.)