My take...
"Why are people selling snakes that are months old, many feedings, and still weigh under 100 grams?" - Most likely, because they are lying about the feed shedule and have been "maintenance" feeding the babies until they can sell them...less feedings = less $$$ = more profit!
"Why do people start their snakes on such small meals?" - Because they don't realize they can take larger meals.
"has anyone directly (not "I heard X can happen) had an experience where they fed a large prey item to a hatchling and it ended poorly?" - Nope. I've had some feedings that I thought may have been pushing it, but all went fine...
"Did I do something wrong or do people severely underfeed hatchlings?" - No, and sometimes.
"Would a snake size up a meal that is too large and have complications from it (have you experienced this DIRECTLY - no offense but your opinion is not helpful here)?" - Not with hatchlings, but I've had adults try and eat a meal apparently too large. They just struggled for a while and gave up...rat never got more than halfway down. Fed the next day with no problems. Never had a regurge or anything like that though.
I start my hatchlings on f/t rat pups...if they don't take that they get mouse hoppers or small adult mice, and then back to trying rat pups after a few feedings of the mice...