Hello,
Looking at the photo of the engine bay of an US 2019 124 Spider below it seems the bonnet lock is still mechanical
So if you manage to catch the opening cable with a hand or a hook (dry cleaning hangers work perfectly), you should be able to open it
I know that modern cars have panels covering the bottom, but it's the engine bay, so hope there's an acces. If not, you should be able to dismantle them from the outside. I don't know if there's a difference in how FIAT run the cable in RHD / LHD cars, perhaps you have any photos of the engine bay or someone could confirm looking at his car. Where the handle is in the cab could also be a hint. Try reaching in the very front, starting from almost the center, slightly to the side were the cable runs and if you can't reach the cable, just move to the side.
Once in your hand, just pull the cable to the ground. I've opened my father's C3 and a Cinquecento this way when the battery died. I remember there was a problem with the C3 however, but can't remember which one. But remember that finally I got in and was able to take out the battery and replace it with a new one.
As for the problem with the key not unlocking the door physically, does the lock barrel turn smoothly? No idea how far it should go in a 124, I'd say at least 45 degrees, perhaps even 90, it depends on the model.
You should also be able to attach the jump starter on the starter motor terminals, they usually are on the bottom of the engine. If so, tnen you should be able to use the remote key fob to open the car