Parental Duo: When a friendly character named Hulk or Caiera is adjacent to Skaar, they all modify their attack values by +1 if not already modified by this effect.

Power of the Oldstrong: When Skaar is adjacent to a square of blocking terrain, if he possesses Invulnerability, he can use Impervious instead, and if he possesses Toughness, he can use Invulnerability instead.

When using Parental Duo, if Skaar is adjacent to both Hulk and Caiera, who all has their AV modified? Since it says "Hulk OR Caiera" would I have to pick one?
No you would not have to pick one; all three characters (Hulk, Caiera, and Skaar) would modify their AV. The "or" is only there to indicate that you don't need to have both Hulk and Caiera... either one will do. But, by that same token, you are certainly not penalized if you happen to have both of them.

Does Power of the Oldstrong activate if Skaar is adjacent to elevated terrain? That blocks movement and LOF, after all...
True, elevated terrain can block movement and LOF. But elevated terrain is not blocking terran. Blocking terrain, and blocking terrain only, activates Power of the Oldstrong.