A lot to unpack here! Let's start with the cheap shots:
I can't believe a publishing house would put that on the front cover if it's incorrect.
Oh... I would. I would indeed.
Okay, now that I've got that out of my system: I think the friction comes from the intersection of narrative and reality. Or, to put it less abstrusely: stories have their own "present." We're all familiar with literary present tense, in which we can say "Hamlet kills Polonius," without having to quibble with the fact that he first killed him around 1600. Gilgamesh and Ted Lasso share an abstracted present as we sit outside of their timelines.
It is this, really, that lets "how the story ends" become enshrined as a phrase of its own (just Google it and you find the titles and lyrics of at least a dozen songs). So to my view, "... how your love story ends" is simply literary present.
The friction you're feeling is because the hypothetical challenge positions you, the reader, as a character in the narrative. To the Hamlet of Act II, "what's past is prologue" (to mix references). The character within the narrative is conscious only of the linear flow of time. This sentence positions us dually as literary critic, knowing "the story," and as an agent within it, poised to enter the linear stream. This gives us a sort of Cubist view of both the timeless and the timed at once, thus your cognitive dissonance. "If you knew, Mr. Hamlet—wait, I mean know?—that it was—is—will be Polonius behind the curtain, would you/will you/won't you—oh dear..." A certain degree of verb-tense confusion is inherent in the premise of the challenge, knowing "how the story ends" when you have still not "begun"—we are to know the end from before the beginning?
So yes, in my view, the literary present saves us from a Star-Trek level of confusion about how the timeline interacts with itself. In fact, I could see using it even in a simpler conditional construction in which we aren't meta-actors:
If you knew how a book ends, would you choose to begin it?
Yes, this strains grammaticality, but I would defend it because Hamlet (spoilers) ends tragically, whether you conditionally "knew" it or not going in. So even though your knowledge is conditional, and you would normally pair it with a past tense, the literary present is unaffected by your condition.
Is the way it's written acceptable?
Well, obviously it was acceptable to the publishers, and it clearly got its point across. This treads into subjective territory, but to my mind, the "punchiness" of the streamlined version is not a mere gimmick, but improves the conveyance of the meaning.
If you knew how your love story ends, would you dare to begin?
If you knew how your love story were going to end, would you dare to begin it?
There's certainly a pleasing assonance in "ends ... begin," which is lost if we stick on an object, "it." And the published version even (surely accidentally) conforms to an anapest meter, while any past tense must disrupt the unconscious scansion.