I am going to answer your questions backwards, due to simplicity. As far as the setting a conditional branch by level, that does not exist. You CAN, however, set a variable that checks the level and set a conditional branch that goes off that variable. I'll post a SS here to show you an example, it's rather simple really.
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As far as your original question, that's a bit more difficult and is going to take some time. I will work on it for you though and see if I can't get you a solution in a few hours.
Hope this helped somewhat.
Edit: Just to check, that script allows you to add and remove party members that are not locked at will, yes? You may consider taking out all members but the one you wanted, and allowing the player to re-add who they want.
Or are you allowing the player to choose which member is the solo fighter? I have several solutions that may work, but they all depend on the situation. Allow me a bit more information if you would.
Edit 2: Heh, heh, I may have oversimplified these, but here are a couple of event based examples you can use. The first one is if you want them to choose who they fight with, the second is if you want to choose for them. Allowing them to re-add party members saves you a load of grief and is only slightly annoying for the player.
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If you are looking to control who is added back to the party, I can do that with variables but it's a lot of work, so just let me know if this works for you. If it does not, I'll post an event for the full creator control.
Edit 3: Eh heh heh, we are going to pretend I didn't post that original second screenshot....