Just shows what ignoramuses they are if they've cancelled the parking charge because nothing you wrote bore upon the charge itself only whether they could hold you liable instead of the driver who they did not know. But it's probably easier for them because without the driver's details and unless they could frighten the keeper into paying they were on a loser. No point throwing money away, I'm certain they've got other more gullible targets lined up.
I'm glad you didn't complain to all and sundry before about the hotel, after all they wouldn't know you from Adam and as you didn't supply the guest's name what did you really expect? And of course if you were the driver and expected the hotel to make this assumption then if they'd contacted Smart then you shouldn't have been surprised if they did the same.
You were wise not to lose focus.
But if you do know something about how parking operates then I'm interested in this in the context of their reply and data protection e.g. does the Mercure website and booking conditions refer to a driver giving their data(VRM) to the hotel or a third party, how is this done, what role does the hotel play e.g. enters a guest's data into their records which they make available to the third party, or the guest does this at a publicly accessible terminal which is only given shelter by the hotel or is it actually operated by them etc?
IMO, although we see this sort of problem occurring occasionally, I'm of the opinion that hotels should include this as part of their reception duties and arguably part of their care responsibility to guests.
But then again, what's a guest? Your posts are careful with words and the time of exit(2am) suggests that the driver was not a staying guest, simply someone attending a function.
By all means PM me.