What does love have to do with being a soulmate, cats? Presumably soulmates
can be in love (assuming soulmates exist at all), but so can people that aren't soulmates. You don't have to be a person's soulmate to be in love with them. Saying, "It's a given" is one of the really basic ways of saying, 'I'm making an assumption here', yeah. Or, more precisely, making the statement that you feel (rightly or otherwise) that the assumption is implicit. But a couple being in love quite definitely doesn't imply that the pair are soulmates (unless everyone potentially has several thousand/tens of thousands/millions of soulmates, which would make it a rather strange thing.), and whether R & M are, is not stated.
If robin was to be imprisoned for life, that would have been akin to being imprisoned for life, not akin to death. By your separation bit,
permanent separation is the only means of separating true love (whatever the blazes that is, which is completely unmentioned in the story), of which death would be (barring afterlife, anyway!) and imprisonment
might be a method of occurring, depending on how the sentence was enacted. There's no guarantee (again, the story doesn't say) that Marion would be forever separated from Robin if he was permanently imprisoned, though. Visitation is a thing.
Regardless, if Robin's actions had justly earned life or execution (though the method of justly earning such would be tricky), I'm not exactly sure I'd call it coercion of
Marion to execute his sentence, if there was any coercion occurring at all. I'm not really sure what just punishment looks like from the direction of the one being punished for wrongful action. Perhaps coercion of a sort, but a rather odd sort.
Though, as a sorta' aside... I don't think the test is necessarily a 'fallacy' (nevermind that that's something rather different, methinks). It just might not have been testing what you were assuming it was testing