1- Opening preparation is in good part skill, because you cannot just learn the moves by heart and get away with positions you don't understand.
Lousy analogy : classical theater is not real theater, because those guys learn their scripts before going on scene, only improvised acts (commedia dell'arte, etc.) are real theater ! How can it need skill to just learn the script and say it again on the stage ?
2- Remember that endgame practice is preparation too.
3- How much of which (preparation / improvisation) ? Guess it depends on the player. Carlsen is said not to prepare much, although obviously he knows more than I do in virtually every opening. On the other hand, Topalov is a monster preparator. I remember a game in Europe Echecs (a chess periodical) where he prepared a knight sacrifice in the Slav around move 25 against Kramnik and won in the complications that followed.
To me it seems that top chess is related a lot (maybe too much) on opening preparation. Of course, to be a top gm you need amazing skills, but there are some games which go on for over 20 theorical moves until the endgame.