There's a join at the bottom edge, I managed to fish it out with a screwdriver and then pull it around. I think it helped that the whole area was doused in WD40 where I'd been trying to free everything up. Might be worth going over the area with some.
When I've had a door lock seize, I've managed to pull the rubber weather strip out from in the inside of the car. This has then allowed access to the striker and lock mechanism, where I could then use a big flat blade screwdriver to open the latch manually.
If you're not too worried about originality, I'd swap both battery cables out for mk4 golf ones. They're much thicker: Untitled by Dave Ackerman, on Flickr
Again (like a broken record) great write up! Always enjoy your Nurburgring reports. Shame your mates couldn't go with you, that's a hell of a daunting thing to be flying solo, good on you for pushing on! We're actually starting to plan our 2024 trip, if you want to try and match dates, could act as a safety net for next year?
Also, love how Nige is happy in every single track photo!