So, if I'm not mistaked, to be able to bring in a Campaign Mission the Elite Rebel Saboteurs (or the Elite Rebel Troopers), the Rebel players must win the 2 Side Missions that "unlock" Rebel Saboteurs (or Rebel Troopers), while to bring Luke (another Elite, this one Uniqe) the Rebels havo to win only 1 Side Mission?

Precisely. And if you unlock the elite version, you can still choose to bring the regular version if for some reason you want the regular ones.

"Rebel Allies", RRG, Page 5:

If an ally has both a regular and elite Deployment card, only the regular card can be used as an ally. If an ally is rewarded to the heroes and the heroes have already earned that ally, they can use either the elite or regular Deployment card.

Essentially if you unlock the same reward twice you get a more powerful version of it rather than nothing at all.