Ouch to the shoe paint!
I can't imagine how gutted you must feel, I'm be traumatised! How about phoning your travel insurance company, say you purchased your bag, and something must have spilled on the case and damaged the bag? (No need to mention the hair spray?) Worth a phone call?
I know it isn't much consolation, but at least the mark is on the bottom of the bag. I'm sure the mark wouldn't be too noticable, I imagine, when it's standing and not too much when you're carrying?
If it's really bothering you though, and you get no joy with your insurance (and if memory serves, Amex used to do an accidental damage/loss cover with items for 30 days - not sure if they still do) then rather than having a go with the shoe paint, take it to a specialist cleaner, and see what they say? From experience, I know I've tried the "home remedy" approach, and have made myself feel even worse, when I've hashed it completely!
I hope you get your *still* very lovely bag sorted!
Good luck!
K