(a) The published price is based purely on the Comex price.
I don't believe that either. Sure coins and small bars are more expensive than spot price. You can still buy as much "larger quantity" gold at spot price as you want. Pete the Plumber may be buying gold coins on Ebay at silly prices and feel his family will be safe holding two Kruggerands in a shoe box but I think we all know better.
b) The price for physical is based on the amount of physical gold that's available for sale
Yes sure, but that supply and demand can change significantly from even a $50 rise or fall in gold price. Most things are for sale at a price.