I was elated to see you are thinking of setting up a decentralized exchange. I've followed the progress of, say, B&C, but my perception is that they have become semi-centralized as they progress from vision to reality. Any interface with fiat will always be a choke-point, imho. I hope you solve it.
I think all 'decentralized exchanges' so far, and there are quite a few of them now, are semi-centralized - even where they use only crypto let alone with fiat.
A truly decentralized exchange between crytocurrencies would be too slow and probably too expensive in fees to be particularly useful for most people even if atomic cross chain transfer tech was built, so its a tough problem.
I also really hope a good solution can be worked out for this.