I don't understand what you mean by that. If the first actor only gets skills 1 through 10, then alphabetically sorting them isn't going to give him skills 17 & 20. I also don't get why this would necessitate updating the positions with other actors. Already, the same skills will be in different positions in each actor's arrays depending on what skills they have. So, for instance, if Actor 1 has skills 5 & 7 and Actor 2 has skills 2, 3, & 5, then skill 5 will be in the 0 position for Actor 1 and in the 2 position for Actor 2. That doesn't cause any problems.
Also, it's not really any more efficient to alias and not redefine. It's actually (slightly) less efficient. The reason you would want to alias is more for compatibility reasons, as otherwise redefining the draw_item method could easily conflict with other custom skill scene scripts.