Nobody really has any idea of how multiplayer will work yet, so it may well be that young Ellie is playable but can't be killed. You might have to capture her or something, like an alternative kind of melee attack. That could work for competitive or co-op, I suppose.
The thing is, the survival pack is all MP, not anything single player
You're clearly wrong, because the Joel & Ellie skins are so obviously for SP.
The Survival Pack
Your best chance of staying alive in brutal multiplayer matches, this pack contains:
- Bonus experience points to give you a head start on your rivals.
- A powerful multiplayer melee attack booster to pack a heavier punch in hand-to-hand combat.
- In-game currency to buy boosters, customizable parts and other items to bolster your multiplayer characters.
- Special customizable items for your multiplayer character.
- Two bonus skins for Joel and Ellie – available once you’ve completed the single player game.
Just because you have to play single player to unlock the skin, doesn't mean it can't be for multiplayer. See Marco Polo or Sir Francis Drake in UC2 multiplayer. This was from the preorder screen and the description that I underlined clearly describes a multiplayer only DLC pack.
In other words, Uncharted couldn't do it because it was T, but I think that brook being M may give Last of Us an opportunity to do so.
This is what I think. Please correct me if I'm wrong.