Yet another Humble Bundle question

No, you won't lose what you already own

No, you dont.

No, just buy the $1 tier.

Nope, you won't lose a thing. If that key has all the DLC for the game, then activating the non-BTA key shouldn't get rid of your DLC. However, if you activate the key that does not have the DLC, then your other key will only activate the DLC. So its in your best interest to activate The Full Package key, to save yourself some hassle.