I don't think you can prove anything; certainly not in a court of law; with the information you have available. So you'll have to take responsibility for now, and then..
- Is there anything going on with the accelerator pedal? There was a recall on them. Is the gizmo installed as part of the recall there, and possibly causing problems?
- Post to other Soul EV internet forums about your issue (Facebook has a decent sized group), so if someone else has the problem, they can contact you about it.
- Assuming you keep the car and think it could conceivably happen again, consider installing a dash cam, with a second camera mounted where it can see the pedals? (Might not work at night, but might be better than nothing.)
- Try to duplicate the problem. Was it a super-short trip? Some EVs have strange behaviors when they're at 100% charge and can't/won't do regenerative braking. (Ford Focus according to http://electrifyatlanta.com/wp/?page_id=25
But I put in an order anyway... Hopefully won't regret it.