>>How would my plugin know the difference between multiple identical close orders<<

The difference is not needed, since on NFA accounts it does not matter which order is closed. Only the close amount matters. When positions are not closed immediately, you need anyway a bookkeeping of the positions in your plugin. When a position closes, then return 1 and pass the fill price on the next close order of the same amount. If the close order has a different amount, just ignore it. Unlike open orders, close orders are repeated until the right amount is closed.