He sounds like a character that would be revealed during a Direct where nothing is going on with anniversaries or release dates of other games, i.e. a random out of the blue choice. Either that or nobody finds out about him until launch.
Regardless, Dillion is a rather low profile character, so I don't see him fitting anywhere with bigger names like Ridley or K. Rool.
Your points are completely understandable. Still speaking of low profile characters, at E3 2013 we got the Villager, Mega Man and Wii Fit Trainer. Wii Fit Trainer seems to me also more like a low profile character and it's even difficult to say that Villager was a big deal. Villager and Wii Fit Trainer were both surprises while Mega Man was the long-awaited confirmation.
I think showing Dillon alongside with K.Rool or Ridley, people would ask themselves: "Why is a pretty unknown character revealed the same day as big newcomers?" Of course, this would fit for every less known character.
A reveal at a Nintendo Direct would however give Dillon more spotlight because he would be the Smash highlight of a Direct. Also Little Mac shows there doesn't have to be a game related character reveal.
Dillon as a hidden character wouldn't be anything he could benefit in terms of rising popularity as big names always keep the smaller ones in their shadows. Not even knowing about the smaller names wouldn't make anything better.
And after all, we know Sakurai wants to give every newcomer a special trailer. Therefore we can at least expect to know almost every newcomer before the release.
In my opinion, both, an E3 reveal as well as a Nintendo Direct reveal would be good in order to make sure, people will notice Dillon. However a Nintendo Direct would be the right way to give him the full spotlight as we wouldn't be one of the newcomers shown at an event but be more THE newcomer of a certain Direct. Revealing Dillon after release seems to me to be a bad idea. It would be like revealing Olimar in Brawl after the release of the game.