the Koh Samui airport authority never mentioned that I could clear immigration at Suvarnabhumi Airport and instead cleared my immigration at Koh Samui itself
If you're doing something unusual (such as going to the city during a long layover), it's on you to (a) research your options and (b) bring up your plans when appropriate with airline and immigration agents.
The authorities were just following the established procedure, they don't have the duty to inform every passenger about all possible alternatives, on an off chance you'd like to do something unusual.
I couldn’t exit the Suvarnabhumi Airport
Did you try to get another visa-on-arrival? Did they refuse you? It's a hassle (and costs money), but should have been an option.