I think practically a merchant would need to choose a trustworthy or a couple of trustworthy traders (bisq would initially choose the highest rated, highest volume traders). It could almost be just a single trader they work with with the option of choosing another if they're not satisfied.
Maybe there could be a way to instantly trade for tethers or some other fiat pegged currency and then have the bisq trader trade the pegged currency for fiat.
Yeah, actually with guys like Bitpay I believe the settlement is periodical, daily, weekly or monthly even to save costs, but the thing is, it's all settled in the fiat amounts at time of confirmation fixed by invoice #, so merchants know what they're selling in fiat values will be close to or even exactly what is settled later.
I suppose very close margins can also be achieved with one or two large, trustworthy traders - but it's still unlikely that traders will be able to close deals quicker than a processor. Bitcoin confirmations, offline times for trader. The stablecoin option on bisq almost sounds like a match for the need. So yeah, interesting to see if someone might take on this btcpay integration. I imagine it would be a preferable solution to most existing processors... but got to first see adoption with btcpay + bisq...