I'll explain how I saw it. The waddle dee would be a floaty kind of projectile, so it would stay in the air longer. In other words, rather than just a projectile, it would also be a kind of defensive attack. So in front of D3, the Waddle Dees would be thrown in an arc, up to down. It could also be useful for hindering recoveries.
So here is what makes it harder to approach him:
1) The waddle dee throw animation would be quicker.
2) It would be a floaty projectile, thus stay in the air longer
3) It would fall in an arc, air to down.
4) The arc would be mid-range. Farther than what Dedede's melee range could reach, but barely.
5) Upon hitting the floor, it would vanish,
6) The random elements like the spikey could remain.
The way to use it effectively would be in the air. to cut animation time when landing. One could also combine this with wavedash to keep distance, possibly. In a sense, it would hinder enemy approach, like Grim would hope for. The mid-range would ease approach by forcing the enemy to guard or dodge. I think he has a different approach though.
I'd like to hear your oppinion on this, but also, Grim, you seem to have an alternative. Would you like to share it with me? I'm interested in how you think we could inspire his move from Arakune. My take seems closer to Faust from Guilty gear.