Encountered a similar problem with my Wee when I installed the Corbin seat. Cable bound up and I could not turn the lock to remove the seat. Working under the belief that engineers always design a secondary access in case the primary one fails, I looked for another way to get under the seat to free the cable and open the lock. Found that I could get to the side panel fasteners by removing some fasteners and dropping the rear fender a few inches. Got the side panels off and was able to reach the end of the cable with needle nose pliers and open the seat latch. Lubing it first with a little whiskey & foul language helps.
Don't know if the same approach will work in your situation, but worth a look.
If that doesn't work, try striking it very hard with a ball peen hammer. Something will open up.