The commitment signature of the message "MWIXNET",
It may be wise to have the message additionally commit to the date of the coinswap (and for an hourly swap service, the hour of day as well).
"description": "The excess scalar encrypted using label \"excess\" ",
Why does this field need to be separately encrypted? And what does encryption using a label mean?
"description": "The range proof encrypted using label \"rangeproof\" (only required for the final round)",
Since the rangeproof is only required for the final node, I don’t think it belongs in the per-round data.
Does the json array type for rounds make sense when its elements will be at different nesting levels in the onion encryption?