Please find the appeal and response files here: https://file.io/YzAlxq6GSnJt.
This link doesn't work for me - can you use Imgur or Dropbox etc.?
I believe the car rental had already told Smart that I am the driver
It's not particularly important in this case as you identified the driver yourself, but the rental company will (or at least should) have nominated you as the hirer. Unless they were in the car with you, they cannot state who was driving.
My appeal was mainly due to lack of signage (no signage on entry, no signage where parked, and not clearly distinguished from a nearby Starbucks parking lot), so I was hoping to go on that. Can POPLA request Smart provides evidence at the time of the offense?
You can certainly put them to proof, and they'd be required to provide evidence that the signage was there - They wouldn't be expected to produce photos of the signs on the day, but they'd need to show that signage was there, i.e. with recent photos. The issue you may have is that Smart have seemingly been brought in after the Google Street View images you shared (I say this as the PCN was issued using Automatic Number Plate Recognition [ANPR] cameras, and I can't see any on Street View.
What is the downside if I appeal and lose and would I owe anything else besides this fine?
You don't owe anything unless a judge says so - POPLA is not binding on you, and if Smart wanted to enforce the alleged debt they would need to sue you in the county court (small claims track). They would no doubt add on debt collection costs of £70, which can be challenged even if you lose, and can claim some limited fees for legal representation (usually £50), and court fees (£35). That's assuming they go to court at all, we don't see all that many Smart claims, I'm not sure how litigious (if at all) they are.
Also, as I mentioned further up, if they've still not sorted their contract issues out, then that used to be a guaranteed withdrawal at POPLA, we can help you with the wording on that point.