Please note I added this to my prior post. I will delete this post after you've read it.
My point was that purely DE will not work. You and TierNolan are apparently proposing a slightly centralized design with a trusted third party. It is well known in literature that TTP and/or probabilistic protocol is necessary.
I think the blockchain acts as the trusted third party here, especially with the interlocks where each side waits for confirmations.
the latest fee scripts are not centralized in any way, though if the InstantDEX doesnt spend the fees that it gets, then the deterrent effect of the fees are gone, so there is arguably a slight centralization to make the disincentives work. However, no central party is needed in the mainstream case of the trades completed. No central party is needed in case the trade doesnt complete (as both parties redeem the fee).
Sure looks like a good practical first version approach
James