Amortentia is an idiosyncratic potion: the sunny color, the goldfish-playfulness of the motions, the smells. But a good potioneer can disguise any elixir. And Snape has always been a good potioneer.
He and Lily often worked on potions together. It would be easy to pass her a camouflaged Amortentia, saying "Take a swig of this, Evans. What do you think?" Actually it would be supremely hard: brewing the draught of infatuation, making it look safe, and-after the disaster of fifth year-getting her alone long enough to swallow it. Maybe he'd disguise it as a hiccoughing solution.
Then: happiness. Although Snape found himself more and more willing to play dirty, both with the Death Eaters and with Lily's heart, he wouldn't hurt her like she'd hurt him. No revenge. She wouldn't experience more than a second of unrequited love before he slipped his arms around her, nuzzling the back of her neck. He'd heard that a kiss under the influence of Amortentia was unimaginably fulfilling, so Lily should enjoy herself at least as much as him.
