One method I have used for bikes where the seat latch is sticking, despite the key turning, is to deliver a karate chop to the seat, in this case just behind where the step in the seat is and in a downward and slightly rearward manner.... whilst having the key held in the unlock position.... assuming the key is actually operating the latch, there is probably too much pressure on the latch for it to undo... the karate chop method gives a sharp downward release of the pressure for a moment and this sometimes does the trick....
Doesn't always work, but it might.....
If this doesn't work, you have to first establish if the cable is still attached to the latch.... if it is then you may need to get at it between the side panel and frame to pull it further than the key will turn it.... again possibly delivering a sharp blow above where the latch is while exerting a pull on the cable....
If it is detached, not sure what you can do, but trying to cut a hole in the rear mudguard under the release lever might be an option.... of course that means wheel off etc...
Hope you are successful.......