This one is a little hard for me to review with so little understanding of cowswap.

Have you been able to test that it works end to end for a trade (not just the pre-signature)?

Yes, I was successfully able to do a trade on forked mainnet. This one does get hairy as I'm not an assembly expert, but I've spent some time looking at the code and seeing what it does and it's basically just taking info from the quote data from cowswap and creating the orderId from that data and matching with the orderId we are presigning. If you check the reference the guy who made the issue had an example and it's mostly copied from that. We can get on a call and walk through it if you would like