MY GAME IS A SCRIPTING NIGHTMARE! I NEED HELP!

Yanfly has already done this for you (if your using VX Ace.)

No limit to your party size, up to 5 character in battle,
and the ability to change party members in mid battle.

Otherwise you should be able to change you battle party
from your menu at any time.

"scripting" had made me think a newer RPG Maker, since RM2K3 doesn't, uh, allow any form of scripting.
Fortunately this can be done trivially with just dialogue options by talking to dudes in your base or whatever if you don't want to figure out how to make custom menus. You just make a room where when you enter it removes everyone from your team but the main character, and you can talk to people to have them join. There are hundreds of other ways to do party switching, but this is an extremely easy one since you seem to be having trouble.

You can't easily detect who's in the party and in what position. But, in the room where the team members get added, each time the player adds a character to their team, you can set a variable indicating who just got added in what slot.

You'll need five variables: Number of Party Members, Character 1, Character 2, Character 3, and Character 4. When entering the room, set the number of party members to 1, and set the first variable to the main character (assuming he never leaves). When adding a character to the team, first increase the Number of Party Members by 1, and then use a conditional branch to detect what the variable is. In the conditional branch, set the either the Char 2, Char 3 or Char 4 variable to the ID of the character being added, depending on whether the number of party members variable is 2, 3 or 4.

Now you have variables telling you who's in each slot. Whee! When using a rock candy, you can call a common event that displays a choice to the player. If your Character 1 variable is variable #41, then the first one of the choices should be, \N[\V[41]], which will display the name of the character whose ID is stored in variable #41.

re: Party Makeup; There's condition branches and page conditions for 'If X is in the Party' and there's a variable to track the size of the party. There's no native mechanism to see the order of the party though.

It's risky, but you can also try running a parallel process that keeps track of the time of the current song. Let's say you have a variable "Music Duration" and you increase it by 1 every frame. When it reaches a certain value corresponding to the end of the song, you can switch to a similar background music track that doesn't have the .5 seconds of silence. Or even delay the song playing until a certain point by using this variable that keeps track of time.

You can also use Disharmony (http://www.uglyhorst.de/rPG%20Maker/patches) to set loop points within mp3 music files.

As an aside to this, I should mention to everyone that Disharmony loop points do not seem to create that music loading lag, because they start the music streaming from a certain point instead of loading it all over again.

I have no idea! Probably!

but man if you're using midis I don't know why this is even happening in the first place; it seems like if the first thing you do after the battle ends is change the music, it should start the new song immediately. Is the song you're playing afterwards an mp3?

Hmm... Before you edit any raw data of your game resources, have you tried fiddling with the point at which you change the music? This sounds like some kind of sound card buffer issue or even a bug in the original XP script coding.

Instead of doctoring the music, try playing a short .mp3 or .wav of silence (as a BGM), then changing the music.

Edit: Whoa, I forgot that this was RPG Maker 2003. Below, Locke is right: sometimes you have to be creative within the engine instead of trying to force it to do what you want it to do. But I'd encourage you to keep messing around with event commands for now. Like trying the Memorize BGM command.