Pokémon Hunters, eh? And looking for Scatterbug? I agree that this could be some sort of Vivillion Forme-collecting scheme of some sort.
As for the Diggersby Trainer, I think it's definitely a hunter, but with the debate over how un-intimidating he looks, I think looks can be deceiving. For all we know, he probably might be very smart, to the point where he'd pull off a trick on the group to make them think he's stupid and therefore innocent on the matter. And unless it's confirmed there's only one Hunter in the episode, I think that he could be a simple grunt, 'cause if he isn't the main Hunter, then all of my money is on him being a grunt.
I actually think that Serena deserves to catch a Scatterbug, even if that will make her, as others say, "a May clone". However, if that will be so, I can only hope for a little twist in the fact that she gets disgusted by it when it likes her a lot, and Scatterbug spends the next few episodes following the group around until Serena warms up to it.