+1
Totally agree. I was in exactly the same position as you.
People talk about fiat holders but we are dealing with a trading scenario and for most it was probably just a question of timing as to whether we were in coin or fiat. There is no such thing as a "fiat holder" in the context of a crypto exchange.
I disagree, when you trade BTC for fiat each coin can have only one owner at a time else there would be an imbalance. eg. for BTC/USD there must be one BTC holder and one USD holder at completion of the trade.
The IOUs created in the trading database can each only have one recipient, and must be honored as they were at the time of the last trade (or rolled back to the last backup snapshot which would be worse) You can't have two people claiming ownership of the same crypto coin or the same fiat which is what you are trying to do by disregarding the holding, timing is everything.