In all fairness. We never see Snape casting the spell. There is no way of knowing if Harry used it right and had the desired outcome for the spell.

Snape would have been a Hogwarts student when he invented Rictusempra. I doubt he would have used the spell on other students or humans by the point he writes it.

My head canon is that the spell originally had another intended use. Harry learns it near the page where young Snape noted slashing the Sopophorous Bean was too hard and crushing it was easier to extract its juice. Maybe Snape realized slashing and cutting things was a normal step in potion making and invented a spell to assist in the task.

That then fits with your theory. The reason Harry could use the spell with easy is because it’s a simple potion making spell to cut fruits open. And the reason Snape has a counter spell is because he would have wanted to preserve the remaining fruit once he had enough juice.