I think there might be a miscommunication between us. I'm not trying to argue that Lucina is her own character due to the physical differences between her and Marth. Rather, I'm trying to argue that those differences, along with Sakurai's apparent interest in giving Lucina her own abilities, made it more feasible for her to be her own character. My point about Dark Pit is that he is inherently near identical to Pit; while not impossible to make him his own separate character instead of a simple alternate costume, there is less to work with and, I think, less reason to distinguish the two.
I'll answer your question regarding Dark Pit not getting revealed in Palutena's trailer with the following answers:
1. The trailer ends on a cliffhanger. In fact, it remains the only trailer that does this.
2. He's a secret character similar to Mr. Game & Watch.
3. He is being teased for a future villain trailer (one could make the argument that Bowser and King Dedede are the only confirmed villains so far for that very reason).
4. Lucina was confirmed in Robin's trailer in order to help hammer in that Chrom isn't playable. I reckon he may have known about the gematsu leak itself.
5. Mega Man, Sonic, Pac-Man, Luigi, Toon Link, and Lucina were simply the sole exceptions rather than the rule (maybe a couple others too, but I feel those six are a near-lock for unlockable, if not a guarantee). Each of these six have specific reasons for their early reveal after all.
6. Sakurai may be teasing at all the hidden newcomers in the game before revealing them.
7. With the six exceptions aside, he is simply not revealing the hidden characters, period.
Besides, just because he has an identical build that we must automatically disqualify Dark Pit from playable status simply because he looks like Pit--and in spite of his unique abilities? I guess then that Sakurai must demote Wolf to alternate skin for Fox simply because they look too much alike, huh? If Wolf could get in Brawl without having to be a Fox palette swap and with his unique abilities and properties, then so can Dark Pit in spite of their similar appearances.
EDIT: I'm still convinced we're getting a playable 8-Bit Mario.