Party Switching would need to be done through an evented scene, probably involving lots of picture events and variables. It is not a built in feature, and so unless you are a good eventer, you will probably need to find one. And not many people still use 2k3 on this forum :/
I haven't used 2k3 for a long time, but I'm guessing no. If it's not a built in option, then it can't unless you create your own battle system through events.
As for duel, that should not be hard. You need to keep track of which heroes are in your party, but once you event that, then all you need to do is remove all party members except the main, and then restore them after the battle.
To keep track of how many heroes are in your party through events, just reserve variables that represent which slot each hero is in by ID. Then, whenever you add a party member, just update those variables.